tube recommendation please

which one should I get first which will give me the best bang for the buck?

Thanks, great explanation on the signal tube versus driver tube. I will double confirm that 12ax7 is the signal tube. I also read some comments saying the rectifier may not have any tonal impact. I just order a pair of CBS-Hytron 5814A (12au7) from eBay based on some comments on the internet. I think I am good now. I will wait for the tube and see how much difference does it make. If the result is positive, then I will spend more money and get some nicer 12ax7, like the Amperex long plate D getter =)

BTW, I got a question in buying tube. On eBay, there are many single tubes. But most of the time people would need a match pair. The test score provided seems to be different depends on the instrument used. So .. if I buy a single tube, would it be very difficult to find one that match? It seems those score reflects the life of the tube. If I got un-matched tubes, does it means one may die earlier than the other? Or it will affect the sound in a negative way?

Thanks in advance.

I bought my unit used and it comes with PrimaLuna brand tube. Actually, they are very good. The whole unit with stock team is a big upgrade from my previous Audible Illusions L1. The AI is very good and a great leap from SS preamp, but I am very impressed with PrimaLuna even with stock tube.
Michaelo, what tube does it comes with your unit?
Detredwings, you get me very interested in the Full Music. I did a google search, it seems it got rave reviews! And price is much more reasonable than Amperex or Mullard NOS tubes. That is the next one on my list.
I still can't find out which one is the signal tube. I will send an email to PrimaLuna to ask.