You may find this helpful. However, remember, that personal tastes may vary.
Joe's Tube Lore
Joe's Tube Lore
Also if you describe the particular tube/tubes ( 12ax7-12at7 etc..) you want to upgrade. Many can give you there experience of different type sound various brands have.Such as "warm vs. analitcal sounding tubes. Each brand has there own sound ...This will help in picking the tube Brand ( or sound ) that you'd wish to try...
FWIW, as I recall the SFL 1 you can use either 6922 or 12AX7a's ( I think you must make a minor adjustment in the preamp). If that is correct you have a lot of options depending on what tone you want. Best to listen to what you have carfully, then come back later when you know what tonal changes you want to effect by changing tubes (or tube types). Jobes Tube lore is fine, but it only covers NOS tubes. For less expensive new production tubes you'll have to do some online research. They have been discussed to death.
A few words of advice:
If you are serious about tube gear and tube rolling, buy a calibrated tester. You will be surprised at the number of disparities in your results versus seller claims you'll find. Another occasional disappointment are shorted tubes you'll identify. Know what you're popping into your expensive gear.
I use SFL-2, but never know anything about SFL-1 or other pre. Recently, I have a luxury to audition some of NOS tube (resonably priced) with my SFL-2 and found:
1. Siemens CCa - very musical, might be a bit "thin" (compare to others) in some system, best resolution. But with problem of microphonics (I have tried 3 pair from different sellers, all came out microphonics, I don't know why)
2. Amperex USN-CEP (white labeled) - warm, focus, less musical than CCa.
3. Amperex 6922 PQ (green labeled) - similar to USN-CEP.
4. Philips Miniwatts SQ - a bit less great than Amperex, but a bit better bass.
5. Mullard CV 2493 - quite similar to Philips SQ.
All have good soundstage, the differences are not very noticeable to me (or I could not describe them in detail), but they are all very good tubes. If your system is a bit dull, CCa is great. Others may add some warmness in your system. Amperex is generally more balance in LF, HF, and Mid. But I am a new-tuber, so these are just my opinion.