About Tubes.........

As far as I know EL34's are interchangable.It's mostly about experimenting with your particular amp and having some idea about what you are trying to improve. I have no experience with your amps at all, with stock tubes or otherwise. But, before I laid out big bucks for NOS tubes I'd do some experimenting with current product tubes. I found I liked the sound of Svet's which are also very dependable (no crashes in 4 sets - fairly long lived). In my amp these took the edge off some excessive brightness which was present with NOS Tesla's (E34L's). Also, don't over look the small tubes - these can have a bigger effect on you sound than the power tubes, and sometimes the right combo of power and small tubes will do more for you than either alone.

It's a no brainer to replace with Mullards if you have the money. However, some inexpensive tubes can sound good. Svetlana and Shugang will be an improvement over basic sovtek or chinese tubes.

Some EL34s will handle a little more voltage than others and you may get 4 or 5 more watts out of them but they should all be interchangeable. Just be careful to watch the bias settings on your amp.

Personaly I think the Svetlanas are the best bang for the buck and maybe a cry'od pair might be even better...