I believe an amplifier being " Class A " is too simplified as far as being an ideal design - as people are guessing /  speculating - consistently in this thread.
There are seemingly several other design considerations, that make for a best possible sounding amplifier design.
The use of feedback, VS:  zero feedback, is one other ultra important criteria.  Solid state amps MUST use some Negative Feedback.  Not necessarily so however, in the best tube amplifier designs.
I have been told, by sharp circuit people, that whereas PASS solid state amps  advertise " No feedback loops ", which is true,  Nelson Pass ( and other ) solid state designs need to incorporate " nested " feedbacks, from one stage to another.
I have had two good friends in the USA who purchased and built Nelson Pass " ACA" or "Amp Camp" kits.  They each did a proper break in, using high efficiency ALTEC or GPA speakers.
After sufficient break - in, ........BOTH of my friends independently decided - to sell - off their ACAs on eBay. 
Their PASS amps could not compete satisfactorily with their existing two stage Directly Coupled tube amps, of special design.
Jeff