Quad ESL-63 and low-powered amps; Sun Audio, Atma

The 63’s have 4ohm load from 6khz-12khz and then an even lower (epdr) of 5ohms combind with -40 degree phase at 14khz.

This indicates an amp that needs to be able to drive 4ohms as it’s best feature, not as a secondary feature. Which means (for tube if you can get it) an output transformer that’s specifically wound for 4ohms use, not 8ohms with a 4 ohm tap.

https://www.stereophile.com/images/66Quadfig01.jpg
" Dropping below 6 ohms below 50Hz and between 5kHz and 18kHz, with minima of 3.5 ohms at 10Hz and 13.85kHz, this represents a reasonably demanding load for an amplifier to handle. Tube amplifiers would be best used from their 4-ohm tap (if they have one), though any solid-state amplifier that more or less doubles its power output into 4 ohms compared with 8 ohms would have no problems driving the Quad."

Cheers George

willemj, not intending to misrepresent your position. I appreciate the clarification. Aside from the issue of clipping, I see then that we will not agree on the question of amps sounding different from each other. So be it.  :) 

Willemj, I also find it hard to swallow that all perceived differences in amplifier sound are due to issues like clipping. My biggest problem with statements like that is the same problem I have with the objectivist camp in general: it assumes we understand why amplifiers sound the way they do to our ear/brain interface. I don't think we know even a small fraction of that. So how can anyone assert that this or that is the sole cause of sonic differences in amplifiers? It presumes perfect knowledge no one has.

Douglas, thanks for an elegant response.

Montaldo, I don't think we need to be omniscient for this - we will never be. Let us not forget that the job of an amplifier is an easy one: it just has to amplify an electrical signal without otherwise changing it. Fortunately absolute perfection in this is unnecessary: there are experimental data on the critical levels of imperfections: how non-flat can frequency response be before we notice, how much distortion and what kind can we have before we notice etc? And fortunately technology has progressed to such an extent that ever since the seventies of the last century it has been possible to produce amplifiers with imperfections that are below the threshold of human hearing acuity. Peter Walker's 'straight wire with gain' has been achieved, and at increasingly lower cost. There have also been subjective evaluations in the shape of blind tests to see if under controled conditions panels of experienced listeners can distinguish properly designed amplifiers from each other (I participated in one). The answer was that these panels could not distinguish them.
And yet there are continuing accounts of audiophiles who claim they can hear significant differences. How could this be? Are the differences they hear real? And do these audiophiles actually aim for neutrality or do they want a more euphonic version of the original (i.e. do we have the same objective and are we, therefore, talking about the same thing)?
When you look at measurements of audiophile amplifiers in Stereophile you can see that many do not measure particularly well, and are likely to colour the sound in one way or another. In short, they will sound differently, since they are not playing the same 'straight wire with gain' game. I presume this is intentional, and if that is what consumers want, it is up to them. I have tried to describe what makes such amplifiers different. My contention is simply that if you want a straight wire with gain, that has become an easy and fortunately also quite affordable target.
Anyway, to end this, I want to post a link to an interview with my audio hero, the late Peter Walker: http://www.meridian-audio.info/public/interview%5B4446%5D.pdf