Nope I heard all kinds of decent tube amps on them, tube amps like the Melos range, the large ARC D250 Servo, Conrad Johnson Premier One and a few others I cannot recall. Also i had a friend who owned the huge Acoustat 8's....which If I'm not mistaken were even bigger than the Spectra 4400's. All highly flawed to my ears...with a certain plastic coloration that we haven't even touched upon yet.
All electrostats have somewhat of a plastic coloration...its because all the panels are made of....drum roll please...plastic! You can hear a "quack" from conventional drivers made of plastic as well. And you can get a zing from metal ones and in general ceramic ones sound "whitish". Materials colorations are one of the biggest issues with speakers and often the "flavor" of a speaker. I find nearly all ceramic driver based speakers unlistenable because while a plastic coloration is somewhat subtle the problem with ceramic drivers is anything but. One of the most "neutral" materials seems to be carbon fiber but again it is still not silent. Paper also has a character, doped or not, but it is somehow consonant with music, unlike metal and ceramic.
One of the biggest problems I hear with conventional, multi-way speakers is when the speaker is using different materials for each frequency range. For example, a doped paper woofer, a metal, ceramic or plastic midrange and then a metal dome tweeter...all stitched together with a high order crossover. You will hear the coloration contribution from each driver, especially if an instrument bridges two or more of those drivers...just awful and will completely destroy the illusion.
The ear/brain can mask/ignore a constant coloration and once assimilated to you don't really hear it anymore. A speaker with multiple colorations that get excited at different times means they will continually jump out and never become repetitive enough to allow the ear/brain to ignore them. They destroy the sound. You want to talk about coloration of a speaker, THIS, IMO, is the real killer of most cone/dome box speakers. Nevermind the boxes contributions, which are also difficult to ignore because they get excited by only certain frequencies and then jump at you from time to time as a thickening of the sound in the mids unnaturally.
Given the number and degree of colorations in normal cone/dome speakers, I am not at all concerned about a small somewhat plasticky coloration in electrostats. I never said the speakers are coloration free...show me one that is.
Some of the efforts from Wilson and Magico do a nice job of getting the box part to be mostly silent...for Wilson it doesn't seem to sacrifice liveliness but with Magico...they sound relatively dead to me. HOWEVER, it doesn't eliminate the cone cry and tweeter ringing etc. that you get from each driver in the system. That is probably why Wilson finally gave up on Ti Focal tweeter for a "gasp" soft dome. I detest soft dome tweeters though because, while pleasant, sound unrealistic to me. They are in breakup from a few Khz...albeit a "soft" breakup.
IMO, a speaker with uniform distortion will sound less artificial because we can mask something that is uniform...as long as its not too high in level of course. Apogees also sound quite pure even though they also have coloration...but it is uniform as all drivers are the same basic materials.
This is one of the main reasons I left conventional speakers and even after hearing the latest and greatest I can still hear how they are "stitched" together when using disparate materials. Or they use really offensive materials like ceramic drivers, which I have never heard sound right...not Avalon, not Marten design, not Kharma (who dumped them now for carbon fiber derived), not Gauder etc. etc.
The best horns are not completely color free of course but it is minimized to below what I hear from nearly all box speakers AND they have dynamics and liveness that I don't hear from any box speaker.