With all due respect, I believe your question has been answered above.
Schiit says use matched pair on both gain and output pairs. Others say not necessarily. Just be aware of impedance mismatches.
Experiment. Rolling is easy with the Freya+.
You don't need links to retailers. They are named in the posts above and any third grader can find them on a Google search.
My advice, start out with the stock tubes (JJs last I knew), get a good fix on the sound with familiar material, then try Tung-Sols, right then left, then move to GE or whatever. Bad impedance mismatches will probably announce themselves.
BTW, one of the original JJs in my unit went south after a couple weeks. Schiit said move it from buffer to gain, I did, and the defect was no longer noticeable.
But then soon after, I rolled the whole batch and am still doing so when the urge strikes.
In my experience there is no easier, cheaper preamp to roll tubes on than the Freya+. Seems to be designed for the purpose. So have at it. Read the posts above and remember nobody will have the same experiences given different rooms, different connected equipment, and different musical preferences. There is no magic answer for YOU. Read the tips above and proceed accordingly. You will know it when you find it.