I currently have a custom built amp which have 6V6 tubes. I think the sound is sooooo sweet that every time I plug in my Strat, my body starts to shiver :). However, recently, I bought an Ibanez JEM7V guitar. This guitar sounds better on my old amp which has EL34's. Now I am thinking about changing my tubes in my custom built to EL34, EL84 or 6L6 tubes. However, I don't know how much it will change my sound... How much will this take effect on my sound clean and driven? The only thing I notice right now with my 6V6 tubes, is that the gain doesn't crunch that much as my old amp with the EL34's. Is this only due to the tubes, or could there be an other explanation? Would it in the end be smart to change my tubes from 6V6 to either 6L6, EL34 or EL84 (or perhaps something else?)?
Changing tubes requires a re-biasing. If you don't know what this means and don't have the proper equipment, then do not try anything like this. For your safety and your amp's safety. Simply being an octal power amp tube does not automatically make any two tubes 'switchable' and 'comparable'. Some amps have a built in bias switch to allow switching between el34s and 6l6s, most don't. The sound difference between the two mentioned is described, amongst other things, as being one of the defining differences between an old Fender Bassman and a first generation Marshal (not the sound you and I associate with a Marshal). Enjoy your two distinct amps! Use each for it's specialty. Get an a/b switch to choose between the two instead of trying to turn one into the other.
My first move would be - on condition the valve bases are compatible - to take out the valves from one amp, and use them in the second. Of course, I'd make a recording of each, to have an A-B comparison. There is going to be a difference in the circuitry between the two, mostly in the pre-amp side, appertaining to eq. as much as anything, so a direct comparison (valve to valve) is almost impossible. Sadly, getting a set of replacement valves isn't cheap - but you'd end up with some spares.