You might want to call Musical Concepts. They specialize in Adcom Mods plus more. Been around for many years and have gotten some very good reviews.
Musical Concepts modified my GFA555 many years ago with great results. I eventually gave it to my nephew who loves it. John at Musical Concepts told me that the Mark 2 version was warmer and more laid back but had less resolution and weaker bass, so he thought it was not an improvement over the mark 1, although some people might like that sound. The question is whether it is cost effective to do a complete mod/upgrade on such an old design, when you can buy a better amp for the price of a complete mod/upgrade. That is unless there is something about the GFA555 you really like and would like to keep but just improve upon.