ADCOM

I had a GFA 555II for a couple years. Bought it used. I never had tuff speakers to drive,so that thing never broke a sweat. I think I sold it for what I paid for it.

The GFA 535 amp was reputed to be the sweetest sounding of the amplifiers, if you don't need the power of the 555.  I agree with Wolf, you should watch for issues related to age.

5802 is a great sounding amplifier, and a pretty good home heating device to boot.

I re-capped mine for safety about 6 years ago.

Had a 5500. A big, ballsy, over the top amp, let down by graininess in the mid and upper frequencies.  I sent it to Stan Warren to be upgraded and bought a Sunfire Stereo 300 to use while waiting.  The Sunfire was not the over the bombastic amp the Adcom was, but crapped all over it in lack of grain, black level, and simply disappearing.  The sunfire was far better than the Stan Warren modded 5500, so I sold the Adcom and kept the Sunfire.  15+ years later, I still have the Sunfire (rebuilt last year).  

I also used a GFP-750 for years, great preamp that I just replaced with a Schiit Freya S, which images better, but the GFP-750 was bit better in being perfectly transparent.  The passive mode in the GFP-750 is the best passive I have heard.  

Have had other Adcoms, GTP-500II, GTP-400, GFA-5300.  All were good units that performed above their pricepoint.  

My main amp is tubed Conrad-johnson Premier 11a, but I have an Adcom 555II I used before C-J. I use it as a backup, I have had it 20 years & NO TROUBLE. Nelson Pass does good work! I love my C-J, but the 555II Is an excellent alternative. IMHO Amps are important, but PreAmps really set the tone. My preamp is an Audible Illusion L3B (brand new, but there are several used on the market)