I don't think that 20 watts is adequate to drive the B+Ws, regardless of the amplifier's implementation. The ones that you mentioned in your original post put out around 70 to 90 watts, and that should be fine. I don't think that you'd shorten the life of a tube amp (or the tubes themselves) by using one with the B+Ws, that was not the experience that I had. It just didn't sound good; in my case, everything just sounded flat. You mentioned that you're moving, so maybe the thing to do is wait for a bit, get settled first, figure out your 2-ch listening space and then look into buying an amp and speakers at the same time.
-Mike
-Mike