Here is a good guide to 6AS7 failures that I stumbled on when I was having tube issues with a different amp. When Russian (and some other) 6AS7's arc over there are small "fuse" links that will break. This makes it easy to see if the tube has failed. The pictures are not the clearest, but trust me, you will know it when you see it.
http://www.atma-sphere.com/support/visual.htmlAndrew
