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.
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.