Don't invest in upgrading the Adcom as several amp parts are proprietary (including output transistors) and are no longer obtainable. You won't be able to fix it if anything goes seriously wrong. I owned an Adcom 5802, the later model, and was unable to get it fixed so had to junk it.
I was told this by the tech who owned an authorized repair center and worked on them for several years. He said Adcom had major inventory problems when it went through financial difficulties some years back.