What are the best drop in subs for a 12AU7??

Thanks for all your input so far. The Mullards I own I bought several years ago on Agon and are labled as 12AU7. I do think they sound pretty good. I am pretty sure though that they are not 1960s Blackburns or anything that old. Probably 1970s possibly 1980s. I have to admit I haven't dug them out to check to see that they don't say 4003 somewhere on them. Any other suggestions? Keep them coming! Once again something that is not overly rare or expensive would be preferred. Thanks again!

You might try the very inexpensive (about 30 a pair) but remarkable jj ecc802's.. great balance between European warmth and American detail. 

I've been doing well with Gold Lion 12AU7s from Cryoset.  Very open, smooth sound and they stay quiet for a l o n g time.

@hifiman5  Do the Gold Lion 12AU7's have good dynamics?   With my Mullard 12AU7's, I can hear the drums but there's no punch.

The dynamics are excellent across the frequency range. Kick drum is kick ass.  I tried many different tubes including the Mullards but they got noisy fast.  I would agree with you that they lacked PRAT that gets the foot tapping.  I buy matched pairs from Cryoset.  The current pair has close to two years on them and they sound great and are still quiet.  Frankly, I purchased another matched pair several months ago so I would not have to go without my system if a tube failed.  Those new tubes are still in the plastic wrap!