Tube Rolling in a SF Line 3

The stock Sonic Frontiers Line 3 pre-amp came with 12 tubes:
5 matched pairs of Sovtek 6922 and
a pair of 6HG8A (Sovtek or Jan Phillips 6U8A?)

I'm interested in doing some tube rolling. I think I remember reading somewhere that just swapping the 6HG8A pair can make a huge difference. Is that true? I've noticed that there isn't alot of good NOS alternative available for the 6GH8A -there are lots of cheap ones (GE?) out there that cost as little as $3.

Also, what about swapping only "some" of the 5 MP of 6922's? Is that problematic? or must you swap all 5 mp at the same time, i.e no mixing of MP's allowed?

So. if you mixed 3 pairs of Sovtek's with 2 pairs of something else with a difference sound, would it potentially sound like one or the other, a blend or something else entirely?

Any thoughts, experience, comments or suggestions on this?