With the Adcom amp, I think you’ll enjoy the tube preamps that lean more on the "tubey" side of neutral. I have an Adcom GFA-555 that has been paired with my GFP-750 for many years and I recently bought an Aric Audio all-tube preamp for another system and I really like how it sounds with the Adcom amp. It gives it a nice lush and engaging tone, particularly in the midrange, that the Adcom preamp lacks. Some may say its not neutral but it just rounds off the edges without losing much detail. I think you’ll be delighted when you put a tube preamp in front go your Adcom and for $3K, especially used, you can get something real good.