Tu-bee or not tu-bee... A philosophical question.

Typically the VT-231 would be a 1940's tube. The 6SN7GT is a later version and shares family ties with the 5692 Military tube I am so fond of.

Sound of tubes are judged differently by different people and in different equipment. If you are able to try the two against each other you can determine what you and your gear likes best. If I had to score these two, I would say the VT-231 is "rounder" and warmer sounding, very fast and clean, but not as accurate as the 5692 or even the 6SN7GT.

You cannot go wrong with any of these, they are all twice as good as anything manufactured today.

It should not be surprising if some of the VT-231 variants sound the same as its 6SN7 proper in a vibration-free environment. The difference is that the VT-231 is a military spec version, designed to minimize microphonics in high vibration settings, but electrically identical. These tubes were originally meant to be used in WWII piston aircraft and naval vessels. Yes, some VT-231 variants sound better (the RCA VT-231 grey bottles far outperforms the RCA 6SN7GT's and the Tungsol VT-231 is, IMO, about the best there is) but, depending on your equipment and setup, there might not be as great a difference.