Hi,
My standard Avatar worked great w/ Reference 3a de Capos, and believe it or not, also worked well w/ the Spendor S3/5s. I've heard the Harbeths, but with SS; didn't get a chance to audition w/ my VAC. Above about 70hz, the 3/5s are perhaps the best speaker Spendor makes (and IMO one of the best small speakers period), but it doesn't have enough bass response for me. If I had kept the 3/5s, I would have bought a sub, but I don't know how they would mate.
Also, at the price of the S3/5 and Spendor sub, you could almost get a pair of 1/2s (perhaps demos). My former dealer used to match the 1/2s and the Avatar, and I've heard it numerous times. Great combo if you like the Spendor sound: Some do, some don't, you just have to listen and decide yourself. Spendors have a fuller (richer?)sound than most speakers, and tend to give more body and depth to instruments and singers than most. As a result, Spendors may seem to lack some of the airy quality of some of their competitors -- Proac, for example. Midrange is really special, bettered only by something like Quad ESLs, but bass may not be as deep or punchy as some like, especially in a larger room. Resolution is excellent. HF extension is fine to me, but those who want speakers that sound like brass players are 6 or 7 feet away in the room would be disappointed with the Spendors. Personally, I prefer a speaker that does a good job of conveying the sound of the orchestra or band as a whole rather than focusing in on small details. That to me is resolution (as opposed to detail), and I feel the Spendor is better than most in this regard. The slightly smaller, 2-way Spendor 2/3 is another option. When I owned the 2/3s years ago, I still went to hear the ASO (Atlanta Symphony Orchestra) sometimes; when I came home after a concert and turned on the stereo, I felt that the 2/3s did a better job of conveying the sound of the ASO from mid-hall than any other speaker I'd heard at that point, although obviously not able to convey its full weight.
Having said all that, I'm now on a Quad ESL kick (tried 57s and about to move on to 63s, I hope). The mids and naturalness of the Quad ESL with a good recording is scary real. I put on a John Lee Hooker live CD recorded in 1962 a few weeks ago when I still had the 57s. I was sitting in the sweet spot, but had turned my head to look at my dogs. When the intro was over and he finally started singing, I was actually startled because it sounded like someone else was in the room with me. Way beyond "goosebumps." Of course, they 57s definitely have limitations also, but that's another story.
Good luck.
TLH