Speaker manufacturers quote recommended minimum amplifier power outputs to discourage people from overdriving low output amps into the speakers thus causing clipping which is extremely damaging to speakers. Valve amps don't clip when overdriven they just start to sound compressed. This is not damaging to the speakers (probably not good for the amp though). There is a lot of nonsense around which speakers work with which kind of amp, if it sounds good it is good. Valve & SS amps both produce an electrical signal to drive the speaker, there is only one kind of electricity.
I seem to recall one review of the P3ESR (think it was Stereophile but I may be mistaken) in which the reviewer speculated that Alan Shaw may have used a Quad Classic amp when developing the speakers because he felt they were so well matched.
Alan Shaw has stated that he quotes a minimum power that is sufficient so people don't feel obliged to go out and buy expensive amps. I've never heard him be concerned about driving them to destruction, and I've been to demos with very powerful amplifiers used. The analogy is you wouldn't buy a car with too little power, but you would buy a car with more power than needed and drive it sensibly.
He's made his views known about valve amps over the years. He has mentioned that he has a pile of Quad amps that he has used for decades and gets serviced. They have also used Hegel for demo purposes. So I reckon he didn't count on people going out and buying expensive low power valve amps.
When I got Harbeth SHL5+ I had a very expensive 20w SET and the bass was woolly and basically rubbish. Bought a Quad 909 for £500 and they came to life. A change to 909 Mono and QMP had some minimal benefit, but Devialet 250 Pro gave them more speed (with less power than the Quad, but still more than enough).
I use my P3ESR with a 40w Audiolab M-One which is fine for low listening levels in my office and with headphones. So I agree with NagraBoy on that, except my unit cost £650 new and also has an excellent DAC. I used to use a 909 with them that came from my main system. Again, the P3ESR come alive in a larger room with more power. With less power they will go loud, but with flabby bass. If you keep the bass sharp down to its limit of about 55-60Hz and add a 300w sub, Bob's your uncle.
It is a crying shame that people invest in Harbeth speakers, which are not exactly cheap, and use an expensive amplifier that they think will make them sound better, but when pushed actually makes them sound worse. I think people, Stereophile included, get a little beguiled by the midrange and don't really appreciate how good the bass can be from the 8" Radial2, even the 5" in the P3's.
