Tubes for Belles 21A

Just got a great preamp with 2 12au7, stock. There's a little popping that I'm attributing to the tubes. They haven't been replaced since new and the piece is 3 years old. Anyway suggestions on tubes? Using preamp with Cary cinema 5, Cary 303 cdp, and merlin tsm-mx speakers.
I don't have an answer for you but I would like to hear what people say. I just bought a new 21A Auricap and Ref. 150A power amp and have considered playing with the tubes. So, I thought I would post and see if we can get some tube guru's inputs.
I just replaced my Belles 21A tubes with NOS Mullards. Guess what was in the Belles from the factory, NOS Mullards. Now that is attention to detail. Great amp, I will stick to the Mullards. The rest of the system is now posted, "A very happy audience"