Chinese, Single-Ended, Class A, Questions.

[DF96]

After a short while I managed to make the SS sound almost the same as the Tube.
Not to my surprise, the boosted frequencies of the EQ looked like the Impedance of the speaker which tube amps like to follow.

Yes. No feedback means high output impedance, which means lumpy frequency response. Match the frequency response and you match the sound. A simpler way to achieve this might be to add a resistor to the SS amp output.

I could make the Tube amp sound exactly like the SS with more NFB but that would just destroy its uniqueness and the reason to have a tube amp.

That may be your reason to have a tube amp. Certainly not "the reason".

 

[istealth]

One more observation, regarding my previous post. So when I ground the output transformers on that tube amplifier (which are floating by default), I get immediately instability and parasitic oscillations at the frequency of 48kHz. The amp still functions and the sound is ok, but on the oscilloscope the amplitude of that 48kHz sine is dominant. Also when I measure the signal between two "grounds" of the OTP, so two "0" ohm terminals, I get a clear size of 50Hz with the amplitude about 10Vpp. The amplitude probably is not important because those are floating outputs, but I am not sure where the 50Hz comes from. The amp is again exactly the same as described in this post, and basically the same as any standard 6SN7/6SL9 + EL34 amp without NFB, described online.