Tubes for a VTL 2.5

I need to retube my 1997 VTL 2.5 Preamp and would like to know what would be some upgraded tubes over the J&J's that are currently in there now. I have 2x6350's and 2x12AU87's in the unit currently. Any suggestions are appreciated.

I am driving a pair of Magnepan 3.6's with a McCormack DNA-225 amp as my other system pieces with MIT cables.

I retubed a VTL 2.5 with NOS Mullards and am very pleased. Upscale audio was very helpful in determining which tubes to buy for this preamp. Mullards have more midrange bloom (which I like)than the stock tubes but are very detailed at the same time (which I also like). I'm a little confused, though, since mine uses 2x12au7 and 2x12at7 tubes. Is the 6350 a substitute for the 12at7? I'm also using a ss amp and magnepans with it.
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.

I do not have experience with 6350's, but have a lot of
experience with the 12AU7 family and its variants. Email
me offline if you wish to chat about them.