How is the phase splitter built? What amp/can you post a schematic?
If it uses 1 tube for both channels it is probably a cathodyne:
cathodynes operate with extreme internal feedback, and I doubt that you will find significant sonic differences between various samples (although that's a mildly educated guess: I have not actually tried and Im sure that people will claim they can hear the sounds of birds chirping 3 rooms over with this one tube, but not the other when they swap it like some users did with the front tube on the 708b. Did you know the front tube on the 708b is hooked up to NOTHING!). Whether different tubes sound different here or not, you *do not* need matched sections when using both halves of a double tube for "left and right" cathodynes.
Depending on the amp, if each channel has its own tubes it could be a cathodyne OR a Long Tail Pair.
Long Tail Pairs have some internal feedback, but not as much: IME different tubes sound different in a long-tail-pair. Long Tail Pairs do benefit from matched sections.
Amps with Long Tail Pairs are a bit more expensive to build (very generally: long tail pair "wastes" some gain compared to an amp with a cathodyne so the gain must be made up for with yet another tube! which very quickly adds to the cost) so cost could be an indicator as well.
My summary:
Cathodynes: dont bother tube rolling, use whatever you got.
Long Tail Pairs: experiment away.
There are a couple other phase splitters out there in tube land, but those are the 2 most common. most of the uncomon ones rely on either a patient owner to balance them manually or lots of feedback.
