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

Amperex 7316 and Telefunken ECC802S are the top 12AU7 tubes you can find.

RCA Cleartops from the '50s work for me. Andy at VTS has an excellent slection of them. Very low noise.

I also just acquired a pair of Amperex Bugle Boys and am looking forward to hearing the comparison to the RCA.

I agree with S23chang. RCA 5814's and 6189 COMMAND versions should also be considered if you can find them.

But I don't know how easily you will find either of those two tubes, S23chang! If you do, you will pay dearly for them...seriously big money! The Amperex 12AU7's [Bugle Boy] or the Telefunken 12AU7's [ECC 82's...European designation] might be suitable for your rig, Mechans. You might also try RCA's premium 12AU7, the 6189. DO NOT LET ANYONE SELL YOU A "MULLARD" CV4003 FROM THE MID-1980's...these are not NOS tubes, and sound nothing like old Mullards! FYI, the Mullard brand name isn't even printed on the CV4003.

I would suggest that you contact Andy at Vintage Tube Services...he the "go to guy" for all your tube needs, especially expert information.

I had 4 Amperex 7316s sold them on ebay few years ago since I don't need 12AU7 for my application. It shouldn't be hard to find but do expect to pay $100 a pair.
Because of lower noise floor, everything sounds better in every aspects of the regular Holland made Amperex 12AU7 with less forwardness.