TL-A 5050 mic preamp, what tube should I get?

Just picked up a used TL Audio 5050 preamp/compressor very cheap, and I want to replace the tube to make sure it will sound as good as possible. I have no experience with tube preamps, so I don't know if different tubes have a noticable different character, or if I just should get a cheap Sovtek or whatever. It's ECC83/12AX7 btw.