Which adcom sounds best?

I went and heard an Adcom GFA-555 and I loved it!
Sounded clean and clear as diamond - uncolored, and with black background.

I have other great amps like Quad, McIntosh, Krell, but I really like Adcom now - go figure.

So, my question is which one to hunt down on used market - since they don't make them any more - :(

GFA-555 or GFA-555 II, or GFA-5500?

5500 is the newer one, no?

Anyone know what years they were made in?

Thanks for your time and expertise on the subject!