What brand strings should I use on my guitar?
You want to use a good quality tube. I have no beef with Sovtek or JJ. The other Sovtek brands like Mullard and EH are just more of the same quality wise to me. I also like a bunch of Chinese 12AX7s. That is, I think I get good service from all of them. Now when you are overdriving them looking for some particular tone, then you are on your own. Get several, and plug them in yourself and see what makes a difference and what doesn't. Worst case: you wind up with a couple spares.