I agree with Bgross. I'm guessing that you have a pair of VTL 225 amplifiers. I have a VTL ST150 so it may use the same tubes as your's. If your amps take 6350 drivers make sure that they are still strong and balanced. There's not much to be gained in rolling this tube since they were all made by Sylvania as far as I know but the triodes need to be balanced to sound their best. The 12AT7 tube contributes most of the character of the amp. My favorite 12AT7 type input tube for the amp is a Brimar 4033. It, along with a Mullard 12AT7 are wonderful in the V1 position but I think that the Brimar has a significant edge in body and drive, especially when in Triode mode. Also, try playing (within reason) with bias to fine-tune the sound of the amps to your taste as they will respond well to changes of a few millivolts. If one or two tubes stray from their bias setting by 6 or 8 mV you will sense that something is off. For the longest time I ran my amp in tetrode but after replacing the drivers it sounded fantastic in triode mode and I can't switch back.
My apologies if you don't have a VTL !