My Quad II amps standardly use KT66 output tubes, which tend to be superexpensive. If my present four aren't all in good shape--I won't know for a while--I'd like to have alternatives to look at. In particular, I have on hand a quartet of 6L6's, vintage RCA, but no letters/numbers after "6L6", just that bare designation. I've already been told that they wouldn't hold up to use in my Dynakit Mk IV's, and maybe the same holds for the Quad II's. But maybe not, since the Quad II's don't try to match the Mk IV's output power. Any advice on this? Thanks in advance.