@reubent ..."The two techs that I’m familiar with who upgrade the GFA-555 are John Hillig at Musical Concepts and Chris Hoppe at Hoppe’s Brain. Both are well respected and get great reviews/feedback. ..."
OP @bobbydd +1 to @reubent’s posts above. Definitely worth serious consideration. Also worth identifying if yours is a MKI or MKII original GFA-555. In any case, these type of techs know what to replace for aging and upgrades where it matters.
I took a chance on investing and fully upgrading one of Nelson’s other designs on the Pass/Forte amp side, not exact - but some of the same lineage. Give those folks a call listed above and ask them more about what they replace, upgrade, and the expected differences and outcome. You might be surprised. Super happy with mine, and really like how it sounds.