Re-Tubing A Sonic Frontiers Pre Amp

Has anybody had any experience re-tubing a Line One, Two or Three? If so, what kind of tubes did you use and what results did you get?
The are many tubes out there you can consider to replace the standard 6922 tubes that comes with your SF line preamp such as mullards, nos, and etc. It really depends on the sound that you're looking to achieve. I recently re-tubed my line two with valvos. The bass is tighter and the highs and mid range are quicker. Hope this helps.
If I were you, I would also call Kevin Deal at Upscale Audio (909-931-9686). He is a true professional that will give you excellent advise (IMHO). He has never let me down.
Retube SFL 3 with Amperex JAN (Joint Army Navy) 7308 "white letter". Detailed, better bass, deeper soundstage, to my ears the "Best". NOTE: Only 2 tubes are needed, no matter if you are using SFL 1, 2 or 3.
Lak and Adrian4r have hit the proverbial "nail on the head!" Good advice!
IMHO, the stock Sovteks with the SF Line series leave much to be desired - a very uninspiring midrange. Kevin Deal offers a full retube package that involves replacing only the back two tubes with first line NOS types, and the rest with NOS Russian types. If you can get them, the Siemens 7308s Deal offers did the trick in my Line 1, turning it into a virtuoso. However, Kevin is big on first talking with his customers to gauge what areas they are specifically looking to improve upon - a conversation well worth the effort (considering the price of NOS 6922s these days). I was considering white label Amperexes until speaking with him - for my needs the Siemens proved a better fit. One caveat: Kevin does sometimes have restrictions on top grade 6922 sales. Obviously, these SF models are totally transformed with a good retube.
Thank you for the advise! Can anyone explain why only the two back tubes are replaced with the NOS tubes instead of all 12 of them?

Thanks in advance; -Jerie
Do you know what a dozen white label Amperexes or Siemens cost . . . that's a pretty good reason. Also, NOS upgrading just the final tube stage provides sufficient effect in this application.