Replacement tubes for Sonic Frontiers Line 2 SE?

It is time to replace the original tubes (Siemens) that came with my SF Line 2 preamp. The Siemens tubes are hard to find (and pricey). Could some of the Sonic Frontiers preamp owners who have replaced tubes comment on alternatives to the Siemens tubes?

Unfortunately all real good NOS tubes continue to rise in price as demand becomes greater,and supply dwindles.The Siemens,Telefunkens,Valvos,Amperex are you best bet for the SFI.THe most important tubes are your 2 gain stage ones and you can use Sovteks,or any other less expensive brand for your remaining 2 pair.
My Line 2 came back from up-grade to the SE level with NOS Brimars, and they were too soft and uninvolving for my taste. So I put a pair of Brimars in just the 2 driver positions (gain positions), and used stock Sovteks in the other four positions. This was better than 6 NOS Brimars.

But recently, a friend let me audition a set of six NOS Sovteks (1970s), and I liked them so much I bought them. They were un-used and from Upscale Audio. The NOS Sovteks are distinctly more liquid with more "air", detail, smoothness, and are more holographic than the stock Sovteks-- and also much more dynamic than the Brimars. I've tried other NOS tubes in the gain positons and did not like the (usually) excessive bloom in my system.

I do think it's a system specific thing, and of course personal preferance, eg I like the stock Sovteks much better than the NOS Brimars-- they are much more dynamic, although a little bit dry, IMO. Good Luck. Craig
I've used RCA tubes in the rear most bank and was very happy. They were relatively inexpensive, and came very close to what I got from Telefunken. Also tried Amprex (not sure what flavor amprex, but I bought them new for $45) and didn't notice a difference between them and the stock Sovteks. I'd recommend sticking with the Siemens. Ya get what ya pay for.
My SF Line 1 is currently outfitted with the same NOS Sovteks Garfish references, plus a pair of NOS Amperex 7308 JAN white labels in the gain positions. After trying several others this combination is the best yet.
I used to own the exact preamp as you have now and I remember the best tubes I had on the gain positions are Siemens E188CC with A-frame (best treble extension), and the Siemens CCa A-frame(best bass and mids).
I tried variants of Siemens E188CC and CCa that does not have the A-rame structure and they don't quite sound as good as the A-frame versions. I also tried Telefunken 6922, Ampererex 7308 white label PQ, Amperex Jan 7308's white label,Mullard 6DJ8s and 6922 BVA logo, Valvo white label E88CCs and I repeat none comes even close to the two tubes I described above. Of course this are all just my opinion on how they sound on my system back then.