CJ Premier 12 and NOS tube rolling

I've recently got a CJ Premier 12 and wonder; Which of the tubes ( 6550, 6FQ7 or 5751) is most important for the sound signature/quality?
I would like to swap to NOS tubes, but where do I start?

Thanks :-)

Sylvinia black plate 5751 is a start.I change out the 6fq7's at the same time. My CJs use el34's so I can't help there.
I am using EI 90's with great results I would suggest speaking to Kevin at upscaleaudio.com He can help you taylor the sound to your liking.