I have Quicksilver V-4 amps and they sound great with all speakers I have used with them. They run cool for tube amps, and it is easy to adjust bias, which I check occasionally and does not usually require adjustment. I have not heard the Mono 120, but would guess it sounds similar.
+1, Hard to go wrong with the V-4's. Had them in SE and Balanced version. They get the tone right, which I think some designers may be losing touch with. I have not had the opportunity to hear the Mono 120's.