at least to my ears, lately it seems more and more tube power amps tend to sound similar to Ss amps.
That's because you're listening to tube amps that have feedback and probably PP outputs combined with single-ended input circuits. That tends to make them sound 'solid state'. Transistors often sound that way (harsh, bright) due to insufficient feedback, same as what you see in tube amps.
The warm, rolled off tube sound as it were.
Most tube amps aren't rolled off in the highs unless there is a malfunction. Some are in the bass, especially SETs, since bandwidth is hard to get with SET output transformers.
always I want a tube amp to sound unlike an SS amp… mostly. as the
result of distortion? of course not. ultrra euphonic? nope. not that
either.
People don't like to hear this, but the primary differences we hear in all amplifiers is due to their distortion signature. People call that the 'sonic signature' but to be clear it really is about distortion. This is simply because the ear converts all forms of distortion into tonality.
The catch here is that the ear pays a lot of attention to tonality that arises due to distortion. There's a tipping point; the ear may actually favor tonality due to distortion over actual frequency response. This is why tube amps with no feedback and arguably more frequency response error due to a higher output impedance might still sound more neutral if distortion is otherwise properly controlled in the circuit.
A peculiarity of the ear that IMO/IME is not well understood appears to be that the presence of the 2nd and 3rd harmonics (in sufficient amounts with respect to the higher orders) can help the ear winnow out soundstage and detail (there are limits to this; distortion of any type also obscures detail). I'd really like to find out why this is so. As best I can make out since you can't get rid of distortion, what becomes important is to have a benign distortion signature; if that happens it seems that the actual THD the amp has is far less important. That said there does seem to be limits in this regard; if the amp has 10% THD its likely to be less detailed than an amp with only 1% at full power. So the take away appears to be that if your THD is very low, it still has to have the right signature in order for the amp to sound musical.
