VTL used to use 6350 tubes in their preamps and then later on switched to 12AT7s. I don't know the reasoning for the switch but I would guess the 12AT7s were an improvement. I have a TL5.5 that uses 6350s and I have tried a few different 6350s with no apparent difference. I have used the VTL branded ones, Sylvanias and IBMs, they all look similar and sound the same. I would suggest concentrating on getting quality 12AU7s and try to find a good deal on whatever 6350 you can find. I don't have any experience with 12AU7s so I can't make any informed suggestions for for them.
Cheers
Jim
Cheers
Jim