Matched Tubes do I need them and why?

You might consider purchasing them from Roger Modjeski at Music Reference.

He selected matched tubes for Atma-Sphere monoblocks I used to have in my system, and they never sounded so good.

Assumming that each of the tube types AX7 and AU7 are one per channel, then yes a matched pair of each is recommended. As to how to get them, if you are buying new, just ask the vendor to sell you matched pairs. They will charge more, but shouldn't be too much.

They should me matched. This should not cost much more. If you want to take your pre to a whole new level, buy NOS which cost more. I would guess the 12ax7 are for the phono, while the 12au7 are for the line. You can buy the most expensive pair for the one you use most.

Thanks for all your help, but how can I tell it is a matched set. Are the numbers on the side to be exactly the same? Some ratings on tubes show Idle and Dynamic db while others show T1,T2 and GM for each. If anyone knows of a web page I can get a good education in that would help. At any rate I am having some fun with tubes. Just bought a pair (hopefully matched) of RCA command's and gonna plug them in tonight.

Cheers,