"it feels like a one sound" I get exactly what he means. This is what I recognized when I got my SET and listened compared to my otherwise very push pull amplifiers.Funny - people describe that about our stuff too.
As best I can make out, what seems to be important in any amplifier is the distortion signature, possibly more important than *how much* distortion it actually has, although the lower you can make the distortion the more detail you can reveal. The important thing in the distortion signature seems to be that the lower orders be in sufficient amount that they can mask the higher orders. Many solid state amps violate this idea by having very little of the lower orders, leaving only small amounts of the higher orders. Because the ear is so sensitive to those orders, they are easily heard, causing such amps to sound harsh and bright, especially at higher volumes. This is literally why tubes are still about 60 years after solid state was introduced to hifi.
I recently built a little 5W/channel tube amp using output transformers. The power tubes are EL95s which are a cute little pentode power tube. I wanted something that was low power, compact (as an integrated stereo amp this one can sit on a sheet of paper with full margins visible) and otherwise as high quality as I could come up with (it was for my bedroom system and also a high quality desktop). It uses a differential amplifier for the voltage amplifier/driver circuit (IOW no separate 'phase splitter'). The output section is ultralinear and class A. A high quality CCS circuit was used for the 12AT7 input differential amplifier, so its CMRR numbers are quite high. So literally two power tubes and a 12AT7 per channel. I gave it two inputs and a volume control.
The inputs are single-ended although this amp is fully balanced/differential from input to output. The other input to the differential input voltage amplifier is used for feedback. The amp's distortion signature is expressing a cubic non-linearity, so mostly a 3rd with succeeding orders falling off rapidly. Since the circuit was quite linear without feedback (the feedback is only used to reduce its gain) the distortion signature is relatively undamaged by the feedback. In comparison to a 2A3 based amp I have on hand, this little amp is better in every way- smoother, lower noise, greater detail, overall things sound more 'real'. Without feedback it has bandwidth past 100KHz...
This is just my experience, but when typically SETs get compared to PP amps, its never PP amps of the same power and quality. Its also never SET amps compared to PP amps using the same kind of power tubes. In past comparisons I've eliminated these variables; IME when you eliminate those variables and also pay attention to the distortion signature, the advantages of SET are eliminated- you can get overall more pure, smoother and more detailed sound with greater bandwidth and power.