I don't know the answer to the question posted. I suspect that differential distortion between tubes and transistors is in itself irrelevant and everything depends on the implementation.

I do know this: You can drive a SE tube amp into clipping and still be able to listen to it. The soft clipping and euphonic harmonic structure is obvious, but not necessarily unlistenable. Driving a SS amp that does not soft clip (another issue entirely) into clipping is instantly obvious and totally unlistenable. Whether this is an issue or not depends on how you use your amps. Most SS amps are much more powerful than most tube amps. If you need 1 watt and you have 100 watts, you have sufficient headroom that you will never clip. if you need 1 watt and you have 3 watts, you will be clipping all of the time.

BTW, if some one can actually address the original question of inherent distortion of tubes and SS, could you also add in class D amps.

Bob