Many of the key variables that differentiate one amplifier from another are not published in the audio industry, at least not widely. THD is, of course, just one small component of the overall performance of a complete system. Its not that important.
Two key things I found in building a preamp are channel separation and channel matching.
It was surprisingly hard, for example, to get crosstalk low enough so that one channel, at high volume, did not bleed through to the adjacent, silent, channel. Simply running two signal wires in close proximity can cause noticeable problems.
For the same reason, grounding can be a big deal.
Proper channel matching and separation are what create that 3-d presentation most folks find so wonderful. Well, the whole system contributes and, yes, the speakers are the key component.