Sugarbrie mentioned the Sylvania 6SN7Ws: The two tall versions sound identical and are absolutely astounding in their musicality, sound stage and imaging. The shorter ones might singe your ear-hairs with their highs(unless your system is laid back in the upper registers). the TungSol 6SN7GT round plates(not with the mouse ears though)have very much the same attributes as the tall Sylvanias and work excellently together. I'm using one each in my Cary SLM-100's. If you switch out your Sylvania(bottom getter from the 40's) VT-231s for either of these: Your biggest improvement will be in depth of sound stage. I suppose you already know to shop for the lowest noise tubes possible, though finding either of these at all is getting harder by the minute. I've tried the Ken-Rad bottom getters and the VT-231s were a bit more natural, though not by much. I've not tried the CBS 5692 Red Base as it's scored consistantly low on every 6SN7 family tube shootout that I've read.
