The thing about many of the amps that ship with 6L6's, but
have a bias switch for changing to EL-34's is that the amp's
EQ was tuned to the 6L6 sound. Sometimes, if you put
EL-34's into a Mesa, Peavey, or other similar amp, the EL-34's
don't sound quite right, since that amp's circuit was designed
around a different tube. It never hurts to try it once, but on
the amps with a fixed bias and selector switch, I think I'd take it to a tech to see if a bias pot could be installed internally.
That way, if the fixed bias is a little cold sounding, you can tweak it to warm up the amp a little.