I would actually look at the Krell 202 or 222 preamps (even an older Krell KAV-280p is nice, but doesn't have the power supply of the 202/22). The Krell's bipolar Class A gain circuits give you a nice sweet sound but still have the slam/punch/immediacy. I would avoid the Krell Phantom preamp as I think they are overpriced and there's nothing much in them (even KAV-280p is better in my opinion).
You could look at some of the Pass Labs preamps. They are Class A and use JFET/MOSFET gain circuits instead of bipolar. This may give you more of a richness in the sound (not that Krell would be lacking warmth), but keep in mind that not all FET circuits are created equal. I have not heard Pass Labs, so I could not comment.
Eric is a fan of Ayre, but I have no idea what that sounds like.