I'd recommend buying just a single Sovtek to get your amp up and running properly, then think about the JJ 2A3-40. There's nothing wrong with the JJ 2A3-40, but they are reasonably expensive to put into an amp that isn't working properly, and they don't make any extra power in the Stereomour.
What I would expect with your bad tube is that when you run the amp, the voltage between ground and pins 1 or 4 on the 4 pin socket with the bad tube will either be 0V or just really low, then the other tube will be OK. Using the dead tube in the amp shouldn't cause any additional damage after your rebuild. When I run into situations like this, I will take a black marker and draw a little skull on top of the glass so I don't lose track of the dud!
