Most anything NOS will improve your pre. I have used seimens, tungsram, amprex bugle boy all were a nice improvement but my vote goes for the seimens
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It also depends on the circuit, but I have also found the Siemens E88CC, Cca, late 50's, early 60's to be the most synergistic and musical sounding in my preamp. If you go with Siemens, try to find one date coded with A0 to A3. I have an A3 that is very magical. Even the Siemens of the same early 60's manufacture sound differently. Good luck.
It's very system depedent, but I would vote for Amperex 7308 white label. I recently purchased a ARC LS-25 Mk I with reference level mod from GNSC (Great Northern Sound Company). GNSC had recommended to the previous owner a quad of Siemens 6922s, which sounded pretty good (and measured well in my TV-7D/U tube tester). We had a shootout between my LS-25 and another friend's SF Line 3 SE. The vote split in the middle. (Two for Line 3 and two for modded LS-25) For the fun of it, I changed the Siemens tubes to the Amperex 7308s, and all the jaws dropped to the ground of the vast improvement over the Siemens, and all hands went up for the LS-25 with the Amperex tubes.
For me, I'm still using the Siemens tube in the LS-25 because I think their more mellow sound matches my system better. (Even though I prefer Amperex tubes in almost all other applications)