I too have had John Hillig modify my Adcom GFA555 back in the late 80's. John does some great sounding mods. Since then there is a new kid on the block so to speak. His name is Chris Hoppe of Hoppe's Brain. I have never in my life seen anyone who is more knowledgable when it comes to Adcom amplfication. His work restoring, Adcom amplfiers has to be seen to be believed. He takes every amp completely apart and dips it in a cleaning solution, has his own brand new input PC boards that he installs, along with new PS boards, caps.....ect. He uses some of the finest parts available. From Vishay, IXYS, On-Semi.......ect. He even sells these as kits for those so inclined. Check him out online under (Hoppe's Brain)