Hello there ... Well, EL 34's can be operated at any current between 30 and 100 mA (35 mA is typical idle current in Class B), and then again the difference between the two, in a guitar/bass amp can be up to 5%, perhaps 10% without any bad consequences. Just use a pair of tubes of the same make and age - that should do it.
If you can measure idle currents with new tubes, and if they are not far away from 35 mA - just don't touch it. If they are much higher (over 40 mA) increase negative grid voltage (all amps have some means of doing it). In fact, operating your tubes at higher idle currents will to some extent shorten their lifetime.
The other way around, if idle currents are below 25 mA, you might want increase them slightly - but if you're happy with how it sounds - why should you?
A slight debalance in tube gains and idle currents might in fact "sweeten" the sound a little bit, as perfectly matched tubes almost perfectly cancel even harmonics, and umatched don't.