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.

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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?