The Adcom line including the GFA-5500 has a reputation for a very clean, almost sterile sound. If you want an analytical sound (many of us do) I feel that the ADCOM pre is a good match, I ran an ADCOM pre with mine.
If you like a warmer sound, pick a warmer preamp.
At this point you have an amp that is about as un-colored as you can get for reasonable money, so your options in system sound signature are pretty open to you in your preamp choices. You might want to decide what speakers you're pairing it with first to create a good synergy.