I have used the GFA 555 with the GFP 565 and while the pair is very good sounding (and the 565 has a great phono section if your into vinyl). But, I will say that when I installed the Modwright SWL 9.0 SE ($1000 used) it really showed me how beautiful sounding the Adcom 555 can be. A tubed pre amp is what I highly recommend. Manley labs shrimp?? possibly. Doge 8 Clarity, Dehavilland Ultraverve III, these are all tube based and around your price range and are very special indeed. Also, all of these will have very low output impedance to drive the Adcom no problem.
Matt M
Matt M