What’s the best tube for a Sonic Frontiers SFL-1 preamp

Look, get a couple different tubes, give them each a twirl and sit back and enjoy; It's really terrific piece. I've since moved to a different pre but I find it no great upgrade over the SFL-1. New piece has more tubes and a different presentation. Not necessarily better, just different... keep the Sonic Frontiers pre & play it til the wheels fall off. I still have mine albeit on the shelf for now. 

Congrats on the buy. I am an original (1994) owner of an SFL-1 still in use today with 0 issues. The original tube, which I still have, was a Shunguang Golden Dragon 12AT7A ECD-81. The only tube I have replaced it with every 6 years or so comes from Upscale Audio, Cryo treated NOS Mullard 6201 Gold Grade 12AT7. I only think to replace it when the system starts to sound grainy. Never the amp, always that one tube long into its lifespan then falls off quick. Next one will be Cryo treated from Kevin's stash of NOS because I just gotta know. Start with the 6201 at the least.

Shuguang 12AT7A (Ei), sorry, and right channel went out in 2013, repaired with no further issues. Ordered the tube upgrade kit from the Parts Connection when they were a division of Sonic Frontiers, use the Pearl small tube cooler but never installed the Mullard CV4024 KQDD/K tube...jumped right over it to the 6201. Try either one or I have a tube never used.