I haven't changed the tube in my F1-X, but I will soon.
I also just put a JJ Tesla long plate in my SWR Baby Blue II. The JJ sounds great, very full and warm. The amp seems a bit punchier in the mids now. Barring any super rave reviews here, I will probably order another for the F1-X.
I would also look over at TalkBass for reviews on preamp tubes. There's been a lot of good discussion on those forums, including a couple tube shootouts.