WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I have. The auditory results where interesting, confusing but to my ears entierly unaceptable with one exception, "OTL" Amplifiers designed to drive ESL Panels directly from the Valve Anodes, however these are specialised beasts and ESL's are not my taste in sound either....
Normal "OTL's" driving conventional speakers (irrespective mostly of sensitivity etc) usually sound initially impressive and seem initially to present more transparence and detail than traditional designs, but I find the sound ewearying and overly stressed and dynamics appear to be severely compressed subjectively (I have many times noted people actually calling this kind of effect "good dynamics", not just in OTL Amp's, I call it "bad dynamics"), plus the sound ivariably seemed to show a wearying edge (I have noted people calling this "great detail" as well).
Never heard them. The only 6S33 OTL's I heard so far where Graaf, pretty middeling performance, nothing outstanding to me. The best OTL's I heard so far where AtmaSphere, but these where massive chassis with ton's of 6AS7's (you can think of 2 6AS7's as broadly equivalent to 1 6S33 in terms of output impedance, dissipation and power) and even these had a certain unpleasant edge in the long run and compressed dynamics, though less so than the smaller OTL Amp's I's familiar with.
I still tend to prefer using well above average sensitivity speakers (around 8 - 10db above the mean 87db/W/m common in commercial "HiFi" speakers) and use lowish powered SE Amplifiers.
I have been wondering what a decent SE OTL would sound like, but the demands of such a beasty in terms of numbers of output valves for even a handfull watts into 8 Ohm is scary....
And good output transformers are surprisingly transparent in the midrange and their LF problems are not as bad as may be expected....
Still, I might yet try another OTL in DIY, but not any time soon.