Russian Tubes or Old "American" Tubes (built where ever)?

wolf, where do you recommend going for the Mullard 12AT7's?

You should check out the GE 6DJ8 . I like these tubes and they will not cost you an arm and a leg. I use all GE preamp  tubes,12ax7,12at7,12au7,etc.  I like the ones from the 50's and 60's the best!

Put the Russian's in and see what you think. It's system synergy that we all search for with all of our equipment/part choices.
What works for you may not be good for someone else.
Consider tubes in audio like spices in cooking. Different tubes will give you a different "flavor".
Is that "flavor" good, better, or bad? That is a subjective choice.
I would suggest, that as your budget allows, buy a replacement set of tubes that you are interested in trying. See if you like them. If so, great. If not you can always sell them and recover most of your original purchase price.
I have a very extensive collection of tubes and can tell you that even within the same family I.E. GE, RCA, ETC, there are many differences in sound.
Depending on what flavor you are searching for will determine your choice.
Most of all, have fun with it and trust YOUR ears.

P.S.  I forgot to mention to do a search for "Joe's Tube Lore". 
He has done a rather extensive shootout of several different tube families. 
His results are a good read and a good starting point. 
Remember, your results will/may vary from his. But Hey, isn't that what makes this hobby fun?

Many NOS tubes are very good, but the can get pretty pricey. At least you are only looking for a pair, which helps a lot. The NOS market can be scary and dangerous if you don't know what you are doing. Pricey, but some shady re-sellers out there.

For new tubes, I have found the Psvane tubes to offer very good quality at reasonable prices. Of course, as always, YMMV.