Home Theater Review contributor Terry London caused quite a stir a couple months ago when he reviewed the $160 Pioneer SW-8MK2 subwoofer, which he claimed was better than his former reference $999 REL T-7 subwoofer. What created even more controversy was his test of a version of the SW-8MK2 that a dealer in Oregon had modified. The modifications included, according to Terry, "a proprietary method to influence and speed up the electron flow of the subwoofer's amplifier."

I found out about this at 8:00 a.m. on the morning Terry's review posted, when a speaker manufacturer called me up to chat about it. "If this guy can speed up electrons, he should be working at Los Alamos, not messing around with subwoofers," the manufacturer said.

The review got me curious about why Terry preferred the inexpensive Pioneer enough that he replaced his REL subs with them permanently (well, as permanently as anything can be in a reviewer's system). I didn't doubt that Terry heard an improvement, but I wanted to figure out why. I also wanted to see if this modified subwoofer, which costs about twice the price of the unmodified version, could substantially outperform the stock model.

Terry and the editors of Home Theater Review agreed that I would run measurements on the stock and modified versions of the Pioneer SW-8MK2. Fortunately, I already had a full set of measurements I made on the REL T-7 subwoofer. Pioneer agreed to send a factory-fresh sample of the SW-8MK2 for me to test, and Sean Scoggin of Stereo Dave's Audio Alternative provided a sample of the modified version of the SW-8MK2.

Pioneer SW-8MK2 vs. REL T-7
Despite the REL T-7's $1,000 price, it's not safe to assume that its performance outclasses that of the $160 Pioneer SW-8MK2. The two subs are about the same size, and both have eight-inch woofers. The T-7 has four times as much amplifier power (200 watts RMS vs. 50 watts RMS), which would give it a +6dB advantage over the SW-8MK2 if all things were equal. However, the T-7's driver appears to have greater excursion and, probably, lower sensitivity compared with the SW-8MK2's driver, so that could reduce or even eliminate its theoretical output advantage.

The REL does have one indisputable advantage, though: It's 70 percent heavier, in large part because its enclosure walls are thicker and thus less likely to resonate. That's a difference I expect would be audible.

There are many possible and legitimate reasons why Terry liked the Pioneer SW-8MK2 better than his REL T-7. The SW-8MK2 is a ported sub, while the T-7 uses a 10-inch passive radiator. Ports and passive radiators work in similar fashion, but the two subs are obviously tuned differently, so they're going to sound different. Which one you prefer may depend on your taste, your room acoustics, the music you prefer, the blend with the main speakers, etc.

My CEA-2010 maximum output measurements show that the REL T-7 has fairly comparable output to the SW-8MK2 at most frequencies, but 6.5 dB more output at 25 Hz than the stock SW-8MK2. I thought that should give it an advantage, but then Terry pointed out that he listens mostly to acoustic jazz, which has no significant content at 25 Hz. (The lowest key on a piano is an A note at 27 Hz, while the lowest note on most upright basses is an E at 41 Hz.) So, his listening tests would not have highlighted this difference.

According to my frequency response measurements, both subs have a low-pass crossover filter response of -28 dB per octave. As you can see in the first chart above, the REL has a flatter response, with a slightly lower -3dB point of 29 Hz, compared with the Pioneer's 31 Hz. However, the two subs seem to operate in opposite polarity (unless I mistakenly had the REL's phase flipped when I did its measurements, but I always check that before I do a sub measurement, so I doubt it). The REL has about 1.5 milliseconds more latency than the Pioneer, which is the rough equivalent of moving it 1.5 feet farther away. The Pioneer also "rings" more than the REL does, as I saw in the tail end of its impulse response.

These characteristics could cause the Pioneer to blend a little differently with Terry's tower speakers than the REL sub does, even if both subs are placed in the same spot. (Note that I said "differently," not better or worse.) It's also quite possible that the REL and Pioneer subs were set to slightly different crossover frequencies; my measurements have told me that the frequency markings on these controls are often inaccurate.

None of these differences would likely have tilted the contest firmly in either sub's favor, so which one will sound best in a certain system is probably impossible to predict. In this case, it depends mostly on which tuning suited Terry's taste and on which sub happened to blend better with his main speakers.

The Stock SW-8MK2 vs. Modified SW-8MK2
To compare the stock and modified subwoofers, I measured their frequency response and their maximum output (CEA-2010 standard). To do the frequency response measurements, I close-miked the drivers at a distance of about a quarter inch; close-miked the ports right at the mouths; then scaled the port responses and summed them with the woofer responses. I marked the positions of the subs using tape, and I did not move the microphone or the table on which the subs were sitting. Thus, both subs experienced almost exactly the same measurement environment.

As you can see in the first chart above, the frequency response curves are very similar. Relative to the stock subwoofer, the modified one has a boost of +0.96 dB centered at 66 Hz and a dip of -1.23 dB centered at 34 Hz.

I then compared the responses of the subwoofers' crossovers, which you can see in the second graph. Knowing that I couldn't precisely match the settings of the crossover frequency knobs by eye, I set the stock model to 12 o'clock, ran the measurement, then adjusted the modified version's crossover frequency knob to match the response of the stock one. The result was a nearly perfect overlap of the filters' low-pass response, except for a peak of +1.32 dB at 58 Hz.

To put these differences in perspective, normal sample-to-sample variation for a product of this sort would typically be on the order of ±1.5 dB, greater than the differences I measured.

I then compared the impulse responses of the two subwoofers, which you can see in the third graph. Again, there are mild differences, but nothing that to my knowledge would create a significant difference beyond normal sample-to-sample variation.

I performed CEA-2010 output measurements at a distance of two meters, with both subs in the exact same position, with the crossover frequency and volume knobs set to maximum. Results are presented in one-meter RMS equivalents per CEA-2010 procedure.

63 Hz: 114.2 stock, 114.5 modified
50 Hz: 113.8 stock, 113.8 modified
40 Hz: 107.0 stock, 106.9 modified
32 Hz: 102.8 stock, 101.8 modified
25 Hz: 87.4 stock, 88.7 modified
20 Hz: NA stock, NA modified

These measurements are typical for an inexpensive, eight-inch subwoofer. They are all within normal sample-to-sample variance for a product in this price range, and for the most part within the CEA-2010 accuracy standard of ±1 dB. Neither sample has a meaningful advantage over the other.

I disassembled both subwoofers and could find no modifications to the amplifier, driver, or enclosure. A few parts are different, but seemingly only in color; the part numbers are the same. One has some yellow potting compound around the jacks to prevent vibration; in the other, the compound is black. One of them has its driver turned 90 degrees relative to the other. All of these differences could be explained by manufacturing inconsistencies.

I e-mailed Sean Scoggin to make sure that I had, in fact, received a modified subwoofer, and he replied, "... what you are looking at is a proprietary modification that was designed to be undetectable to a person who does not know exactly what they are looking for. My late boss before me and myself have worked very hard to come up with modifications that could not be easily copied or detected, as that would give us quite a few advantages."

He further responded: "For a retail $156 sub that has an eight-inch woofer and 100-watt amplifier, a +1dB boost to +2dB boost at the right frequencies actually makes a lot more difference than you would think, as bass is often cumulative and has multiple frequencies that come out at the same time. We have found that, many times, you do not want to boost the bass past a certain amount, as your bottom end tends to roll off faster, which is exactly the opposite that we wanted for this subwoofer. It seems that your measurements, even though they appear to be subtle, actually corroborate what we and the reviewer all have heard when this sub is modified. When modified, the sub is noticeably louder on the bottom, extends further (has more a more solid bass response on the lower frequencies), better texture and a decreased upper bass hash."

It must be pointed out here that the SW-8MK2's amplifier, like almost all amplifiers built into subwoofers, has equalization and limiting built in. If Pioneer had wanted to get an extra +1 dB around 66 Hz (assuming the benefit of the extra output outweighs the likely increase in distortion), its engineers could have simply dialed that in with the EQ. For the manufacturer, at least, no fancy mods or tweaks are necessary to achieve an extra couple of dB more output at a certain frequency.

Considering all of the above information, I cannot confirm that this subwoofer has been modified. I cannot, however, say the subwoofer has NOT been modified. I am merely stating that I was unable to measure any differences that were the clear result of a modification, and I was unable during my visual inspection to identify any alterations that were clearly the result of a modification. All I have to go on at this point is Sean's word and Terry's subjective impressions.

Why did Terry's subjective evaluations note a difference between the modified and stock subs? Many potential reasons can be found above, with slight differences in crossover setting being my guess as the most likely reason. Ordinary sample-to-sample variation and different amounts of driver break-in are other possibilities.

And it is possible there's something I'm missing about the modified version--although to believe that, I would need a description of what the modification is, as well as a technical evaluation of some sort to document its effects. Claims of effects such as "decreased upper bass hash" don't persuade me, especially considering that Terry set the subwoofers' crossover frequencies to approximately 45 to 50 Hz, which would mean the subs aren't reproducing much upper bass.

One important thing to consider is what else the combined $320 cost of the SW-8MK2 subwoofer and the modification could buy you. For example, you could buy a Hsu Research STF-1, a somewhat larger, $309 subwoofer that has about +7 dB more output at 31.5 Hz and a whopping +12 dB more output at 25 Hz. These are Hsu's measurements, not mine, but our results are typically within two dB or so; I use the same measurement technique, and I've actually collaborated with Hsu to make sure our results and techniques are similar. I've also worked with SVS to make sure I'm getting CEA-2010 results that compare with what others are getting.

Bottom line: Might a $160 subwoofer subjectively outperform a $999 subwoofer of similar size and configuration? Sure. Can I recommend a modified version of that $160 subwoofer in which I can find no clear evidence of the modification? No.

Editor's note: We gave Sean Scoggin of Steveo Dave's an opportunity to see Brent's piece before publication and to respond if desired. Here is his response:

"Thank you again for reviewing our subwoofer modification of the Pioneer SW-8MK2. We did have a few additional observations regarding Mr. Butterworth's review of his findings on the measurements of the unmodified versus modified subwoofers. In major audio sites or magazines, it is not uncommon to see a reviewer's impressions and observations on the sound of a particular product not correlating with the measurements of that particular product. Often the person conducting the measurements will be puzzled as to why the listening reviewer really enjoyed the sound that he heard from the product. Microphone technology has advanced a fair amount, but it still is not nearly as sensitive as the human ear. We routinely run into people who have adjusted their systems with their 'room and bass correction' microphones and find that they are often not a positive change. In fact, the sound often can be artificial and strange sounding after the 'extraneous bass' has been removed. We have also heard, and I'm sure you have heard, any number of products that measured quite well, but upon listening to them, it becomes immediately apparent that they have severe, obvious deficiencies in the sound quality and performance.

There are many esoteric sound enhancements that many reviewers and audiophiles swear by that are not necessarily explained in full, as to how they work. Yet many listeners and reviewers, some of high reputation, can hear the difference in their systems when they use them, even though they don't fully know how they work.

As to Mr. Butterworth's theory as to why Terry liked the modified subwoofer as compared his previous RELs, that assumption is flawed in a couple ways. Firstly, as we have found, many subwoofer's crossover points, even in expensive subs, are not absolute--meaning that they are often "leaky" and allow a certain amount of the frequencies beyond the crossover to come into playback. We have heard a decent amount of actual voices come through certain subs when the speakers are off. These were not inexpensive subs. Secondly, many audiophiles know that, when you set up subwoofers with a system, different subwoofers will perform better at particular crossover frequencies depending on the characteristics of the subwoofer and how it integrates into the system. The assumption that every sub will be at its best performance at the same frequency with the same pair of speakers discounts the fact that each subwoofer make and model is unique. After two different subs are dialed in to their optimal crossover level with a system, one sub will sound better than another at its best with the system.

This modified eight-inch subwoofer does not go down as low or produce as much bass as a good 10- or 12-inch subwoofer, but we have actually tested it on electronic bass music and home theater low-frequency effects. The bass was significantly improved in depth, texture, and amount on some fairly low-frequency tracks. We have many satisfied customers who tell us the same thing. These are not just "acoustic jazz" subwoofers.

While you get more of a visceral impact from a 10- or 12-inch woofer on certain low-frequency instruments, such as a timpani or large organ/synthesizer, the majority of music that people play is not in this range. For most people, they are looking for a subwoofer that is natural, musical, and blends well into their system. They're not looking for a subwoofer that shouts "Here I am!" in an artificial manner. The mark of a good subwoofer is one that projects bass into the soundfield but is invisible itself, while positively adding to the soundstage, fullness, and solidity of the overall musical picture. Mr. Butterworth's measurements do not account for those qualities, nor do they account for the combination of carefully tuned multiple bass frequencies, which in his estimation are miniscule but when combined do make a substantial difference to the listener when listening to the modified unit. Again, thank you for the review."

Additional Resources
• Visit our Subwoofer category page for more subwoofer reviews.
• Pioneer SW-8MK2 Subwoofer Reviewed by HomeTheaterReview.com.
• REL Acoustics T-7 Subwoofer Reviewed at HomeTheaterReview.com.