Be careful what you write: it might come back and haunt you, and maybe with frightening rapidity. Last June, I stated that, 'the PrimaLuna ProLogue One is now my "affordable reference amplifier".' I spoke too soon. I hadn't reckoned on the ProLogue Two.

From a few feet away, unless you have good-enough eyesight to read the legends, you cannot tell the One and Two apart. That's because they're 95 percent identical. And, at £999, the Two costs a meagre £200 more. I suppose it's like choosing between 1.2 and 1.4 litre versions of the same car.

Like the One, the ProLogue Two is an integrated stereo valve amplifier built on a 7.75x11.5x15in (HWD) chassis, but weighing slightly more at 37.5lb. Its fit and finish are of the same high standard as the One, with what seems like a baked enamel coating in a dark sapphire blue/green that's almost black from certain angles and under certain lighting conditions. The valve cage is in the same finish, again using banana plugs to connect cage to chassis. And that's a stroke of cleverness that all veteran tube amp users will appreciate, especially tinkerers who like to change valves with any frequency. It's just so obvious. Makes you hate amps with cages held by 16 self-tapping screws.

At the front, the Two uses the same knob layout for source select and volume, high quality items with the volume pot from no less than ALPS. Again, as with the One, the black front panel is finished to perfection, the power on LED fits its aperture snugly, the side-mounted power on rocker switch has no free play. However much it hurts to admit it - and I'm the last person who wants to see Bentleys made in Germany - you must dismiss any thoughts that the Chinese have not yet achieved consistency in manufacture.

Beneath the cage are the four input and driver tubes, and then the primary difference between the One and Two: the output valves. While both amps' front-ends employ two 12AX7s and two 12AU7s, the Two replaces the One's EL34s with KT88s. The result is only an extra 5W or 10W per channel, but more about that anon. Behind them are the same massive, well-made and quiet transformers. Both amplifiers feature the company's Adaptive AutoBias automatic bias control, which constantly monitors performance and alters the bias accordingly, extensive protection circuitry, four line inputs with access via gold-plated sockets, a lack of line outputs, and multi-way terminals with separate binding posts for 4 and 8 Ohms.

Your £200 pays for more than the difference between EL34s and KT88s. It also buys higher quality Nichicon and Solen capacitors, some circuitry tweaking and higher quality superfast Philips diodes instead of the simple rectifier found in the One. In addition to the slight increase in power, the specs remain virtually identical with a frequency response of 20Hz-30kHz +/- 0.5dB and THD of less than 1% at full power, with an S/N ratio of 89dB. Input impedance is lower at 65k Ohm instead of 100k Ohm, while input sensitivity is identical at 300mV.

So what this gets down to is simple: does the extra £200 make a real difference? We have to factor in two main areas of influence. Most obvious is the change in parts quality, and there's certainly a sense of improved refinement and precision, independent of the valves changes. Any of you who have played the 'designer components' game know just how a canny designer can fine-tune or 'voice' an amplifier with judicious use of superior parts, however small their electrical values.

But it's the change of valves that is most intriguing. Why? Because PrimaLuna only extracted slightly more power from the KT88s than the EL34s, so most of the time you won't be hearing a power-based difference. Rather, we're back to something that most of us haven't messed around with in years: debating the sonic rather than the power differences between valve types.

Some of you will recall 20 years back, there was a fracas amongst valve fanciers regarding the EL34 vs the KT77, and similar battles amongst the 845 vs 2A3, and KT88 vs 6550. It never really was about power, especially when you admit that we rarely tax our amplifiers by driving them at or near their limits. No, it's about the valves' actual sonic character, and even the uninterested would immediately note that EL34s are slightly warmer, more lush, while KT88s are more robust and commanding, and certainly more 'modern-sound' in the lower registers. These two amplifiers from PrimaLuna do not alter that relationship, and if you already have leanings toward either tube type, then you need read no further.

Immediately, that tells you that EL34s are more 'classic valve' in their demeanour. Which suggests that choosing between ProLogues One and Two isn't simply the matter of having the extra £200. For some of you, especially if you're in love with slightly bright speakers of limited bass performance, the ProLogue One may be the wiser option. And believe me: the extra power doesn't make an awful lot of difference in practice, although the ProLogue Two certainly strikes you as more 'ballsy' and gutsy. But how much of that is the signature sound of the KT88, and how much of it is the extra wattage?

I ran the ProLogue Two in The KK Entry Level Reference System (see sidebar), with intriguing results. There actually were times, especially when I was playing vintage mono recordings such as Judy Garland's and some Nat 'King' Cole, both on Capitol, when the ProLogue One seemed better suited: more silky, less attention-grabbing. Then I'd put on an early Columbia mono, with their characteristically 'darker' sound than Capitol's, and the sharpening-up aspects of the KT88 came into play.

So, too, with modern stereo recordings, including new CDs from Kings of Leon, Joss Stone, Jools Holland with Tom Jones and Green Day. It was reminiscent of vinyl fanatics in Japan, who keep separate cartridges for specific labels. I found myself preferring the ProLogue Two when the music needed something to grab it by the lapels and shake it up. The new Kings Of Leon CD has this overall moody, bassy, almost murky feel, and the KT88s whipped it into shape; through the ProLogue One, there seemed to be a loss of fine detail. But it favoured fragile, less textured voices.

It could go either way: the crystal-clear warbling of Alison Krauss sounded more natural through the ProLogue Two, while the more artificial-sounding Judds CDs - similar voices, different production values - were happier through the ProLogue One. It made me think of reviewing cables; it was that unpredictable.

However, there was a deciding factor: versatility. The ProLogue two simply cared less about what speakers you hooked up to it, and it handled the Wilson WATT Puppy System 7 with a confidence that almost made it a workable package. The oddball nature of the LS3/5A bothered it not at all, and it worked wonders with the bass potential of the PMCs.

If I had my way, one of these would be carrying M-O Valve Co KT77s, but that's a tube now so rare as to be not worth worrying about...even as a replacement valve for Radford STA25 Mk IVs which used them as standard. That remains my fave tube, and I've always been unable to make up my mind about the EL34 vs KT88 as I never thought of them as comparable.

So here's a tip, should a demo in the shop not prove to be a simple arbiter: bring along your speakers if the shop doesn't have them in stock. It's the only way you'll know. The differences aren't subtle, but neither are they mutually exclusive. Damn: it's like having to choose between Pepsi and Coke. Well, not quite. But you get my drift. The ProLogue Two stays.

Pistol Music 020 7971 3909

SIDEBAR: The KK Entry Level Reference System
With the slightly embarrassing admission that my last reference budget amp only stayed in place from June to December, to be replaced by its slightly more powerful sibling, the KK Entry Level High End System is almost complete. The rules for inclusion are simple: the products must be in current production and no single item can sell for more that £999. So the ProLogue Two just makes it. I remain utterly committed to valves, and am still searching for the arm/cartridge/turntable package that floats my boat, but here's what I use to judge everything else.

Unchanged for years is the EAR 834P mm/mc phono stage, which now sells for £715. It remains the best phono stage I know of below £1000. The PrimaLuna Prologue Two is my amplification of choice, while the CD player is the Musical Fidelity X-RAYV3, at £899. The system is played through PMC's sublime DB1+ mini transmission lines, circa £650 per pair depending on finish. It's the best thing I've heard in an LS3/5A-sized cabinet without it actually being an LS3/5A. (With all due respect to Alan Shaw, he's full of shit.)

Not including turntable until I find my dream budget package - I use a front-end dearer than all of the above combined - the cost of CD player, phono stage, amp and speakers is just under £3200. No, that's not cheap, but then this isn't and readers of this magazine are way past all-in-for-£500 systems. My goal, with stands such as the Tallis from IF and some decent cables, is to assemble an LP- and CD-playing system that will thrill and delight anyone for under £5000, right down to the mains plugs.

Not being exclusionary, I also recommend the following alternatives, for either size, cost or taste considerations: the ProLogue One, Unison Research Unico or Musical Fidelity X-150 integrated amps, the Quad 99CDP CD player, the Pro-Ject Tube or NAD PP2 budget phono stages, and the Wharfedale Diamond 9, the Spendor S3/5SE and Harbeth HL-P3ES-2 loudspeakers. And when I find that elusive, killer analogue front-end, all-in for under £999, we'll try to give the entire system away in a competition.