I've personally found that 6L6 vs EL34 made fairly little difference. Toss 6L6's into a Marshall and it'll still sound like a Marshall - heck, many of their amps came with 6L6's from the factory!

That's not to say there are no differences, just that the differences are subtle and overshadowed by everything else in the amp. Typically 6L6 has a more solid bottom end and EL34 has more midrange harmonics, but again the amp has more to do with that. My most prized amplifier in my collection is an original Rivera Knucklehead 55. Stock it came with EL34's, but it has fat bass and the most beautiful clean Fender chime I've ever heard, even compared to actual Fenders (no doubt because Paul Rivera worked at Fender for years).

One big upshot with 6L6's is they use less heater current, so your transformers might run a little cooler. Might not be noticeable on your amp, but on an old Carvin amp I have it's much cooler.

You might get more of a tonal shift by changing preamp tubes. Long plate vs short plate, JJ vs Sovtek, 12AX7 vs 12AT7 vs 12AU7 vs 5751. Generally you can toss them in without having to change anything else.

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