Which NOS tubes on my ASL 1009 for more warmth?

I was planning to spend approximately $400 on a quad of Mullards and a bit less on a quad of 12au7's...but that depends on how much difference they make. Where would I see more noticable improvement? I bought the amp used I'm pretty sure the caps were upgraded by an ASL technician.

FWIW, you can probably get a fair change/improvement with different 12AU7's compared to the stock Chinese tubes, but before I went there and before I would spend any big money for NOS EL34's you might buy some of the best new production EL34's, such as SED's, and see if that is a big enuf improvement for you. At least they are a great benchmark tube, sonically and ruggedness. Once you have those in place you can do some simple experiments with 12AU7's to find a good match.

Most of your sonic improvements should be with the 12AU7s. You can purchase Mullard EL34/6BQ5 New Production for $25 (matched pair)and save $350. Evaluate your system as Newbee suggested and determine the 12AU7 that best match your system. Plus, you have a lot more 12AU7s to choose from to refine sonic characteristics of your system.

Thanks for the info guys...what are SED's?.

SED is the brand name used by the original manufacturer of "Svetlana" tubes. The current SED's and old Svetlana's as essentially the same tube. They were made in St Petersberg. Around the year 2000+/- this manufacturer lost the use of its name (Svetlana) in North America and adopted the name SED. Its brand logo changed from an S to a winged C (=C=).

If you do some research for user comments here and on Audio Asylum's tube forums you will find that some fairly experienced users consider them amoungst the finest new production and some consider them as good as most NOS. I've been using them for about 15 years and have not been disappointed and have never had a premature failure. I find them tonally neutral. If anything a tad warm compared to some other popular tubes and they might be similar to Mullards, but I've never paid $400 to find out. :-)