Adcom gfa-8500

I think you mean 5800, maybe??? Sonically, I think the 555 and 5800 are quite similar, I preferred the 5800, seemed to have more body, upper mid bass/treble - but it could be different mosfets and power output or speakers (weren't same speakers or room when I listened).
As far as rebuilding, if it currently works then unlikely you will have any issues. I would maybe test the small value input and/or pre amp caps - those went first on my 555 and most every other cap was within spec (changed them all thought to be safe, while it was apart)
With 8-pr TO-3 on each channel, you might could use it as a welder? Good luck, i'm a little jealous if you get it.