However, it is 6sn7GTB therefore not to much choice to play with, unless try not GTB but that, as i understand, not recommendedThere are plenty of 6SN7 types out there, and in a preamp I would expect that a GT, GTA or GTB could be used. You might want to check with the manufacturer on that. The big difference with the suffixes is how much voltage the tube can handle.
6SN7s, FWIW, are an excellent tube (the only bad ones being the modern Russian types) and so I would not expect that tube to be the issue, unless its microphonic or something like that.