Different methods, of necessity -- I'm largely retired at this point and do only the occasional consulting gig, so I no longer hear much outside of the occasional show or visit to a dealer. So I've compared some amps formally here -- besides the A21, off the top of my head, Yamaha and Crown pro amps, an old Hafler, a Vidar, an Emotiva -- none of which are exactly state of the art (the Vidar is the best of them, followed by the Emotiva) -- and otherwise I have to rely on memory of the amps I've listened to over the years at home, in the studio, and so forth. So I've never had a chance to A/B it with something like a Pass, never mind something like a VAC!I would be interested to read what other amplifiers you compared it to (apart from the Parasound) and how you determined any differences.
When I have an amplifier here, I level match and do a comparison with an A/B switcher that I built for the purpose, using both short-term A/B listening to circumvent aural accommodation and long-term listening to circumvent the shortcomings of short-term auditory memory. Then I listen to a variety of tracks that I've found useful for evaluation and with which I'm intimately familiar. So I'll listen to choral and orchestral music, and piano -- the most demanding and revealing -- and also smaller chamber works. I'll supplement that with some jazz and rock, which have their own demands. I find that the difference between good amplifiers is subtle, and while some things stick out right away (the Parasound, for example, has very rich midbass), others are subtle and require long-term listening (forex, a cymbal on the Parasound sounds like noise, while on the AHB2, it sounds like, well, a cymbal -- it rings). Listening is on my Tympani IVA's, which present the amps with an easy non-reactive load but are inefficient so likely to force them into Class B or even clipping and are among the more revealing speakers -- more revealing than most dynamics, less than 'stats (which however are a very reactive load so perhaps not the best for amplifier comparisons). (To put this in perspective, the Parasound, which is a high bias AB amplifier, would with the Tympanis go into Class B at something like 94 dB SPL.)
Finally, if a difference isn't obvious, I'll seek outside confirmation, either by asking someone to listen and getting their unprompted comparison, or by reading what others have heard, e.g., looking at reviews. Typically it matches up -- occasionally I find that I've fooled myself!
That said, I'm not a critic and I don't have an opportunity to hear a wide variety of equipment under controlled conditions as I did in the studio. So while I think I can do a valid A/B comparison if I have the gear here (I've been doing some interesting DAC comparisons lately) I haven't heard a lot of what's out there today under good conditions, as opposed to at shows and dealers, where it's hard to determine what's due to what.
In any case, my characterization of the AHB2 as the cleanest amp I've ever heard is based on years of experience with a large number of amplifiers, but shouldn't be taken as some kind of formal review, it's just my impression -- though if you look at reviews, you'll find that others have independently reached similar conclusions.