I used to own some VTL trioded amps. Did a little experiment. I also had a Spectral DMA50. I loaded the VTL with a power resistor, and tapped the output with some divider resistors so the whole thing was exactly unity gain. Fed that into the Spectral. Feeding Quad ESL-63's.
My purpose was to figure out how much was lost in the VTL, which I considered the better amp. It had more space, 3d effects, smoother, more dynamic, more nuanced, more musical. The Spectral was a good sounding SS amp, but not equal to the VTL. The Spectral was subjectively about 2/3 as good as the VTL. I wondered if the VTL let 99% of the music through or 50%. So I thought taking it in and out of circuit I could get a handle on how much better things could get. Or if the VTL left little on the table.
I did not expect to fire that series arrangement up and hear the Spectral sound exactly like the VTL. Which is what happened. The spacious dynamic musical 3D sound issued forth from the Spectral amp. I later reversed position of the amps, and you could put the Spectral in or out and hear no difference. The wonderful sound of the VTL was a euphonic coloration. The Spectral could exactingly reproduce those effects if fed the right signal.
I switched a few times, listened over days. In time I did find the sound with the VTL feeding the Spectral balanced a little different than just the VTL. Bass was a bit less, and treble a touch less. I knew from measurements the output impedance of the tube amp interacted with the Quad's to raise the lower couple octaves a bit, and a resonance in the output transformer at 26 khz raised the top octave some too. Over time those were audible, but not obvious. Otherwise the Spectral could fully mimic the wonderful coloration of the VTL triode amp. So much for tube amp superiority.