I have a mesa 22 using 2 EL84 and a friend has a mesa 50 using two 6L6.
He says he can hear the difference. We have the same five preamp
tubes, I'm using GE El34 he's using RCA Black Plate 6L6. We set them up
the same as the controls are identical.

His can get louder by a factor of one I'd say i.e. his loudness can
double mine BUT at low volumes and even cranked they sound so identical.

Yet he claims his is creamier and less compressed. Anyone here able to
hear the difference in these output tubes?