VT231 is simply the military designation for 6SN7s. one and the same tube, ultimately. GTA, GTB, WGTA are all rated at 450v, I believe. Personally, I find the GTBs the worst of the lot, much preferred IMHO are the earliest ones: 6SN7w, 6SN7 GT, -WGT, and lastly GTA. About the only good GTBs are Tung Sol and perhaps a few select RCAs. There's many better sylvanias certainly than the later GTB stuff. (See above). GE isn't generally regarded as one of the choice NOS tubes to have, though certainly it's probably the equal of the chinese or Russian modern ones. Ones to seek out, IMHO: Ken Rad, Sylvania, Tung Sol, National Union, the earlier the better. Some of the 1945-52 Sylvania Tall-boys (bottom getter, tall envelope) and CBS/Hytron tubes are basically the same tube as the vaunted Sylv. vt-231, just w/o the military marking. Fortunately, there are a lot of good NOS tube choices in the 6SN7 family, and a fair amount of stock out there trading hands. have fun,
-Ed
-Ed