I am by no means an expert , and no electronics whiz.. Just a hobbyist who loves music, but here goes. . .
SET = Single Ended Triode and seems to represent a kind of sub-culture in Audio. Amps based on SET tubes like the 2A3, putting out just 3.5W per side, usually operating only in Class A, are thought by many to give a sense that the performers are present right there in the room with you -- a dimensionality and wholeness to the stereo image that many feel is lost with push/pull, solid state, and higher power. Hence, SET presence.
The down side is that these "flea power" amps require very efficient/sensitive speakers, and the highs and bass may be sacrificed for the very "present" midrange. Complex orchestral works may have to be foregone in favour of smaller works, like jazz.
Anyway, the point of my original post was that I felt the AVA gear was giving me that sense that the musicians were present in my living room, in a way that many would think was only possible with a low powered SET amp.
The AVA gear, of course uses tubes, and it may be that, with their high power, it is able to operate in Class A for a lot of listening. I'll leave that for the engineers to answer.