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.