Hi,
I'm using a VAC Avatar w/ my de Capos. 27W in triode, 60 in ultralinear. I find that triode is fine, and preferable, w/ female vocals, most jazz and small classical groups. With the stock tubes, I almost never ran in UL because I didn't like the sound as much. Now that I've switched to NOS Mullard tubes, I find I like UL a lot more, especially w/ rock or symphony music. My point is, I feel like the de Capos benefit from the extra power when listening to such music, and I now mainly listen to the VAC in UL when I know I'm going to listen to symphony or rock music. (Contrary to popular belief, the VAC is not switchable on the fly, and switching while it is powered up can damage the amp.)
By comparison, when I had Spendor 3/5s, I felt I never had a problem w/ the VAC in triode at 27 W, that it was always enough power, even though those speakers are a lot less sensitive "on paper". The difference is that the de Capos go significantly lower in frequency response. I think that the 3 db point is rated at 44 Hz, but the UHF review I saw said that they roll off slowly, and have real energy down into the 30s (I believe the 24 db pt. was in the 20s!!) With that kind of response possible, I think the 3As benefit from the extra power. I don't know how the Manley compares w/ an Avatar, but I can tell you that the Avatar has the ability to generate some real authority w/ the de Capos, esp. in UL, in my 13 by 15 ft. room.
Terry