Would like to see it measured...Nice! I own a 535 and 5400 and enjoyed both for many years.
According to this Ken Rockwell review, which includes plenty of measurements, it's a killer unit:
https://www.kenrockwell.com/audio/adcom/gfa-555-ii.htm
He reviewed the MKII version, but it's likely the original (which has a more vaunted reputation) measures very similarly.
I was also struck that the 555 he reviewed measured notably better than the 545 and the two 535s he also measured.
Only issue I'm aware of with these old Adcom amps is that they lack speaker protection.
In fact, aside from a lack of speaker protection (and the mechanics of the power switch, which apparently has a habit of going south), I would think this amp would tick all the boxes for the subject of this thread - it provides gobs of low-distortion power and an argument can be made that in terms of specs it's close enough to modern state of the art to be similarly audibly transparent.
@restorer-john , I hesitate to even ask, but would you agree that this is one of those amps that "could challenge or approach current state of the art amplifiers"? It seems like it is, yes?
The only historical knock I've seen on this amp is a series of subjective reports that it sounds "dry," "etched," or "unrefined." But surely that kind of subjective claim wouldn't disqualify an amp at this forum, yes?
My own experience with the 555 - both into Klipsch Forte and into Quad ESL63 - did match the "etched" and "unrefined" top end description - The Quad 405 I had at the time sounded a lot better on both speakers... so I got rid of the 555... my guess is a bit too much distortion...