I know this is not really what you're asking but I have a VTA SP-14 and just replaced the 4 6sn7's with 7n7's (you'll need a base adapter, readily available on ebay). A much more open and detailed sound with no etching of the treble. The bass seems a tiny bit looser but still powerful and tuneful. Very impressed!
6SN7GT VT231 TUBES
I love the old VT231 tubes!
The best sounding 6SN7GT ever...Sylvania, Raytheon, RCA, Ken Rad Etc. They may still be found, if you have $$$!! Where can I find true NOS VT231s? Any favorites that I missed?
(TS never made a VT231 - but their old black glass tubes with oval micas sound great!)