Matched Tubes do I need them and why?


I have purchased a tube pre amp that has two 12aux7 and two 12 au7's for tubes. I would like to upgrade these an am wondering, do I need matched pairs of each? or do I just need to by new ones. I can get new for about 20 bucks, but have been quoted 110 for matched.

Please let me know what ya think and my apologies if this has been discussed before. Not much time to surf these days.
Thanks for your help.
ghost_rider
Yes, you can get a decent pair for $20. Just buy from a reputable dealer: Upscaleaudio.com, echohifi.com and many more. tubesandmore.com used to match basic tubes for something like $1 extra.
Thanks for all the responses. The unit I am referring to is a P 307 Grant Fidelity http://grantfidelity.com/site/P307. In answer to your question I want the two channels to sound the same and correct. How do I ensure I am getting matched pairs. Are there specs on the boxes showing testing results are the same for each tube?

Again thanks for all your help.
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.