I've used a pair of Russian "Rocket" logo tubes in my preamp for a while now. I got them from Upscale Audio, but you can actually pick them up for about 1/4 of the UA price on e-bay (I recently picked up some back-ups). Just make sure they are the true "Rocket" logo on them. The Russian logo's determine the manufacturing plant over in Russia and for some reason the one's with the rocket on them sound best and don't go microphonic as easily. You need to research the Russian tube factory logo's on-line to identify the proper factory name (Voskhod, Kaluga).
http://www.ominous-valve.com/russtube.html
Enjoy
http://www.ominous-valve.com/russtube.html
Enjoy