Mullards are very nice sounding tubes, I really like their bold low/mid sound. If you can't find them (for a decent price), I can recommend you
RFT ECC83 - 70's East-German - sound a lot like Mullards, great quality
Philips/Miniwatt ECC83* - 70's Holland (!) - also very Mullard like (sometimes identicle), a bit less top end
Telefunken ECC83* - 70s German - nice balance, not as bold on the lowmid as Mullard, superb detail
Tesla E83CC* - 70s Czech - very close to Telefunken
Siemens ECC83* - great balance, Telefunken like
* = many times there are high quality versions available. Telefunken/Tesla/Siemens named them ECC803S. Philips used to add SQ (special quality) to the print. These are even lower noise.
I really dislike Sovteks (harsh, lack of detail), Chinese tubes (gritty, blurry, haven't tested the Mullard copy), and infact most US build pre-amp tubes (lack the sonic quality of European tubes imho). The better part of my collection came from Ebay and tested them myself. But watfordvalves.com is a great source as well.
ps. If you have the time, it might be worth experimenting with european ECC81 (12AT7) and ECC82 (12AU7) tubes. The ECC82 are lowest in gain and sometimes lack a bit low end, but can solve noise/headroom problems. ECC81 in many cases offer great sonic qualities with less gain then a 12AX7. With great results I've used several of these by Telefunken, Mullard, Siemens, RFT, Philips, etc.
99% of the time they are drop-in replacements, sometimes you have to check how the filements are connected. And in some designs these tubes can't be replaced with each other because it was designed specifically around one type.
Good luck
Maarten
