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.
No sir, there is no impedance switch. That's for weaker amps or girlie receivers that have no jam. It'll be fine with your 4 ohm speakers.
On the other hand, you might want to consider the age of the 555. Shine a light inside the amp to see if there are any bulging or leaking caps, as a 30+ year old amp can develop issues.
I was an Adcom dealer long ago, I agree the 5802 was by far their best amp. It had that “French dark roast” character of the rest of the line but with a refinement not in the others. I think Audio Amateur had a regulator board that improved the 555 in much the same direction as the 5802, but never heard a “triple nickel” with it installed.
My first experience in high end audio 25+ yrs ago was an Adcom CD player, Preamp and a 100 w per channel Adcom and 200 w per channel Adcom bi-amping a pair of Definitive Technologies BP 20's it was an amazing sound for the money. I still use it today for stereo in my home office 25 yrs later no regrets no need to upgrade. I'm currently building a home theater in our new home & old habits die hard. I Auditioned some Sandy Grosse creations the Golden Ear Triton 1 and surround speakers for our 9 channel surround ATOMS system. Already purchased the last OPPO UDP 205 I could find & 4 Goldenear Invisa 650 ceiling speakers. I've been consider ADCOM again for AMPS but it seems they have fallen behind in development compared to other brands. Any feedback regarding the multi channel ADCOM amps such as the GFA-7605, 7607, 7705 and 7707 would be appreciated. I understand the 7705 and 7707 need a forklift basically to carry and will run hot and make the power meeting spin like a ceiling fan. Considering other better options any recommendations?