Having owned the AA Capitole my experience was that it performs better direct with some amps than with others. That being said, I also feel that over performance is better with a good preamp. Due to the very warm sound of the AA, I would be go with a solid state preamp (possibly with the exception of an ARC preamp which doesn't add too much warmth, if any). Most other tube preamps I don't feel would be that good of a match.
If you are looking for a very good preamp that is very reasonably priced, you may want to consider a Jeff Rowland Capri (<$2,000). I have owned the Capri and it is a very good preamp. I currently run a tubed preamp (Aesthetix Calypso), but don't think I would recommend this for the AA Cap as my first choice. The neutrality of the Capri should be a pretty good match.
FWIW I do not have any experience with the 3 preamps Georgemg refers to above, so I take his recommendations at his word as an owner of these preamps and an AA Cap.
The AA with the right amp/speaker combo is a pretty nice combo though.
