I went up to Great Northern Sound to listen to the Avalon Ascendants in the last 10 days, and can now answer my question myself. The short version is that I'm buying a pair, though not without a bit of uncertainty. I've gotten used to the Harbeths over the last few months, and they're lovely...very easy to listen to. Hard to get an offensive sound out of them (unless they are paired up with really problematic equipment, I suppose). However, the Avalons do two things I really like, and kind of miss with the Harbeths: They do holographic imaging about as well as I've ever heard, and they present a lot of sensuously satisfying detail in a relatively laid back manner. (They also have deeper bass.) I doubt that they will be as forgiving as the Harbeths are (although I was surprised how much they got out of a couple of torture-test CD's I used for the audition). (How I react to that issue is the part that makes me a little nervous--it seems that there is a bit of a price for increased detail, and I'm not completely certain where I stand after the Harbeths.) The tonal balance of the Avalons reminds me of the Vandersteen 3a sigs I used to have, but the upper mids and highs seem better laid out, cleaner, and richer. To be specific, one of my torture test discs is an EMI recording from the 60's of Barbirolli doing Delius. On the Vandersteens, the strings sounded dry, grainy, a bit muddy, and generally unpleasant. On the Avalons, I heard the orchestra laid out in front of me more clearly, and the strings sounded fine. Caveat however: I haven't heard the Avalons in my house. We'll see. But the equipment Steve Huntley used to demo them for me was similar to what I use at home: Modified Wadia front-end, and Joule and BAT tube gear, with Cardas cable. (I've got the GNSC Wadia front end, Joule pre-amp, and ARC VT100 Mk III on the 4 ohm tap, with Cardas cable.)
I'll report back for those that are interested.