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.
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.