It really depends on how much you're willing to spend on replacement valves, when I decided to buy some new preamp valves for my Marshall amp, I felt justified on spending about $150.00 on a set of three Gold Lion ECC83 (same as a 12AX7) valves because the amp deserved a set of decent valves to sound it's best, the Chinese made 12AX7 B valves I had in it weren't quite up to par and sounded a bit weak.
You could try having a look through all the valves that Evatco stock.
http://www.evatco.com.au/product-list/