Caveat: I have not really followed this thread.
My assertion is usually weasel-worded as "most SS amps operated within their linear region driving most typical speakers will sound the same". Lots of ambiguity because I have not listened to, let alone measured, a large sample of amps connected to a similarly large sample of speakers any time recently. Treat all that follows as opinion.
As an aside: I respect Kal's opinions, and he has certainly opined that he can hear differences among amplifiers. In some cases I have had no problem picking out amplifiers as well so am not hard over on the "all amps sound the same" camp. But it's complicated...
I suspect few of us would have a problem distinguishing a SS amp from a tube amp. The differences, measured and heard, are large enough to be clearly audible on most speakers. One time I designed an OTL tube amp with differential stages and all that jazz, including feedback to (try to) keep it from blowing up speakers if an output tube shorted, and managed to get halfway decent specs out of it. The result? "Too SS-like." I made one of the input stages single-ended again, restoring most of the tube-like distortion characteristics, and then folk agreed it sounded more like a tube amp should. The high output impedance of a typical tube amp, coupled with the fixed-tap output transformer, makes them pretty load-sensitive. One reason I have always felt on ESLs many (most?) tube amps offer gorgeous midrange, somewhat poor highs, and poor bass (where the load is usually pretty high, a mismatch for the transformer).
SS amps generally have low enough output impedance and sufficient frequency response (i.e. the feedback loop can be closed over the audio band) that they will drive most speakers with aplomb. Unfortunately there are a lot of speakers out there that present an "interesting" load impedance. Speakers with wild excursions in impedance magnitude and phase (I have seen <2 to >100 ohms, and with phase changes well over 90 degrees) can represent a very difficult load for an amplifier. ESLs tend to drop dramatically in impedance at high frequencies (the panel is one big capacitor, after all, and of course the transformer is in the picture as well), and that is where feedback falls off as well, so the amp's output impedance increases just when you want it lower. There are stability issues and such that can cause strange things to happen with some speakers.
The gain structure and noise floor of the amp is in play as well; one of things I (many people) found years ago was, in blind testing, an amp with a higher noise floor was readily distinguished from another amp. We did a test with two tape (yes, 1/2" tape at that time) loops playing the same music selections using a couple of big SS amps (have forgotten which, Krell, Threshold, and Levinson were in the store at that time, among others). The speakers I've forgotten but I think were either Magnepan MG-20's or B&W 801's ca. 1982'ish. We might have run with both speakers, too long for me to recall. We inserted a 2 s silence between the selections on one tape, and went straight from one selection to another on the other. The selections were a mix of music but did not have very quiet (silent) passages. Gain matched the channels, natch. With the 2 s gaps, people easily picked out the "noisier" amp, and commented how it "filled in" gaps between piano notes, drum strikes, and such. Without the gaps, nobody was able to tell the two amps apart. IIRC the noisier one was class A but had fairly low SNR, like 80 - 90 dB unweighted, whilst the other amp was something like 100+ dB.
To me the differences in amplifiers are really the difference in how the amp plays with a particular pair of speakers, and of course how close to exceeding the amp's linear range you are. Horn speakers generally require less power but their high sensitivity means more susceptibility to noise (hiss). Panels, ESL and planar-dynamic, tend to be lower impedance and lower sensitivity so push an amp closer to its linearity limits but noise floor (SNR) may be less an issue. And so forth.
FWIWFM - Don
My assertion is usually weasel-worded as "most SS amps operated within their linear region driving most typical speakers will sound the same". Lots of ambiguity because I have not listened to, let alone measured, a large sample of amps connected to a similarly large sample of speakers any time recently. Treat all that follows as opinion.
As an aside: I respect Kal's opinions, and he has certainly opined that he can hear differences among amplifiers. In some cases I have had no problem picking out amplifiers as well so am not hard over on the "all amps sound the same" camp. But it's complicated...
I suspect few of us would have a problem distinguishing a SS amp from a tube amp. The differences, measured and heard, are large enough to be clearly audible on most speakers. One time I designed an OTL tube amp with differential stages and all that jazz, including feedback to (try to) keep it from blowing up speakers if an output tube shorted, and managed to get halfway decent specs out of it. The result? "Too SS-like." I made one of the input stages single-ended again, restoring most of the tube-like distortion characteristics, and then folk agreed it sounded more like a tube amp should. The high output impedance of a typical tube amp, coupled with the fixed-tap output transformer, makes them pretty load-sensitive. One reason I have always felt on ESLs many (most?) tube amps offer gorgeous midrange, somewhat poor highs, and poor bass (where the load is usually pretty high, a mismatch for the transformer).
SS amps generally have low enough output impedance and sufficient frequency response (i.e. the feedback loop can be closed over the audio band) that they will drive most speakers with aplomb. Unfortunately there are a lot of speakers out there that present an "interesting" load impedance. Speakers with wild excursions in impedance magnitude and phase (I have seen <2 to >100 ohms, and with phase changes well over 90 degrees) can represent a very difficult load for an amplifier. ESLs tend to drop dramatically in impedance at high frequencies (the panel is one big capacitor, after all, and of course the transformer is in the picture as well), and that is where feedback falls off as well, so the amp's output impedance increases just when you want it lower. There are stability issues and such that can cause strange things to happen with some speakers.
The gain structure and noise floor of the amp is in play as well; one of things I (many people) found years ago was, in blind testing, an amp with a higher noise floor was readily distinguished from another amp. We did a test with two tape (yes, 1/2" tape at that time) loops playing the same music selections using a couple of big SS amps (have forgotten which, Krell, Threshold, and Levinson were in the store at that time, among others). The speakers I've forgotten but I think were either Magnepan MG-20's or B&W 801's ca. 1982'ish. We might have run with both speakers, too long for me to recall. We inserted a 2 s silence between the selections on one tape, and went straight from one selection to another on the other. The selections were a mix of music but did not have very quiet (silent) passages. Gain matched the channels, natch. With the 2 s gaps, people easily picked out the "noisier" amp, and commented how it "filled in" gaps between piano notes, drum strikes, and such. Without the gaps, nobody was able to tell the two amps apart. IIRC the noisier one was class A but had fairly low SNR, like 80 - 90 dB unweighted, whilst the other amp was something like 100+ dB.
To me the differences in amplifiers are really the difference in how the amp plays with a particular pair of speakers, and of course how close to exceeding the amp's linear range you are. Horn speakers generally require less power but their high sensitivity means more susceptibility to noise (hiss). Panels, ESL and planar-dynamic, tend to be lower impedance and lower sensitivity so push an amp closer to its linearity limits but noise floor (SNR) may be less an issue. And so forth.
FWIWFM - Don