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?
Changing the 6HG8A pair will have little effect. The biggest sonical improvement can be made by swapping the last pair of 6922's (at the rear of your amp).
This was a suggestion of Kevin Deal from Upscale Audio; the guy is an expert on tubes and Sonic Frontiers.
Try matched NOS Amperex 7308 in these positions, keep the Sovtek's in the other and you will hear another amp at moderate costs.
Agreed. If you want to further experiment, stick to that pair of 6922s.
Agree with Peterb, just those two tubes made a big sonic improvement.
Many thanks. Just what I needed to know.
Changing the 6HG8A pair will have no effect. As mentioned only the back two (2) tubes are affected by the sound of the SF Line 3. I have found the Amperex 7308 JAN's or USN to sound superior, of course this is debateable. I have found the Amperex 7308 PQ's less desirable. Changing those back two (2) tubes was worth the effort, with the Sovtek's hand I found the nearset lake and made ripples in the water, may they rest forever in peace. Good luck!
After changing the significant pair of tubes, you can add a tunable tweak: try a pair of Shun Mook tube resonators.
By rotating these ebony caps you can choose for extra warmth or focus. Great stuff !
Amperex green label 7308s in the rear outside pair elevates the Line 3's performance considerably. This particular tube fleshes out the rather lean character of the stock tubes and eliminates some glare/brightness in the upper mids/lower treble. Dynamics and resolution are also enhanced. I bought mine from Upscale Audio's 'cookie jar', available only to those who have purchased their units from Kevin. I've also tried NOS Jan Phillips 6HG8As. These added a nice warm tonal balance, however these tubes also collapsed the large, airy and detailed soundstage provided by the stock Jan Phillips 6U8As. I've just retubed my Line 3 and currently have a MP Jan Phillips 6U8As, MP Amperex 7308 green labels in the rear outside positions, and 4 MP 70s vintage Sovtek 6922s. The latter are only slightly better then current production Sovtek 6922. A bit smoother, and a little more robust in the lower octaves. So if budget is a consideration current production Sovteks would work fine.