I have been contemplating going to the "dark side" (Purify based amp) on my main stereo. Do I still need the masking distortion of my MOSFET amp after resolving the payer and DAC issues? Still needed due to less than optimal source quality? (It does measure .003% @ 1W, 1K or better as that is about my test residual) This led me to thinking about the various reasons tubes, MOSFETS, BJT, or Class D sound different.
These are very generalized attributes as I have experienced them. I have owned and built tube, bjt, and MOSFET amplifiers. Owned class D ( cheap for the garage)
For any technology, the execution matters probably more than the technology. Personal preference matters. This is NOT a better or worse, just thoughts on differences. I am sure there are additional attributes. It would be nice if we could quantize them with how our brain interprets the sounds so we could rely (design to) more on objective measures than subjective. I am assuming competent design and components.
Tube amps have a rolled off top end due to transformer output. Fewer ultrasonic artifacts. We sense ultrasonic and they can cause tweeter ringing and IM issues.
Tubes may have more even order HD than odd. It may be higher providing some masking. Our brain favors even low order.
Tubes have higher but steady noise floor. Again, masking a lot of ills.
Tubes tend to "extended decay" as reviewers say, ringing is the reality. Euphonic masking?
Tubes, for the same measures steady state power, tend to sound more dynamic. Sufficient power supply or less compression?
Tubes tend to have much lower DF so deep bass control is loser.
MOSFETS have less compression with amplitude than BJT. More linear transconductance.
MOSFETS typically had higher bandwidth than BJT and way higher than tubes. Possible differences with IM. Or TIM as the fad in the 80's. No longer a big difference with BJT.
Any technology, Differences in IM with gain. IM being non harmonic our brain does not like it.
Any technology, insufficient dynamic power supply current causes compression and higher distortions.
Solid state, and maybe tubes too, the more parallel outputs, the lower the current in each and the more linear. ie, less compression.
Dynamic power supply current. Here is where budget amps fall flat. Cap arrays about 4X calculated with very low ESR make a big "compression" difference. Dips in the rails can have nastier effects to the IPS and VAS than the outputs.
Class D used to have serious load invariance issues. Combined with switching power supplies way too small. Not sure that is true anymore.
Class D used to have a pumping noise floor. Now so low, maybe irrelevant
Class D tends to drive low Z loads easier. BJT and MOSFET stability get difficult under 4 or 5 Ohms.
Tubes miss-match of the transformer output and load raises both linear and harmonic distortion. Never seen a 2 or 3 Ohm tap. Tough on widely varying load impedances.
Differences in clipping behavior. Inherent but implementation can mitigate somewhat.
Architecture differences, local vs global feedback ratios, Miller vs dominant pole compensation, differential vs se IPS and VAS, and on and on.
FWIW, I have also considered going back to a tube on my desk just for fun. I do love my giant 2W Schiit amp though. It's huge improvement over the Creek is what got me thinking I can use a cleaner amp in the main stereo.