Re: EL84 or 6L6 power tubes
6L6's are more HiFi, and offer higher headroom as well. EL84's sound smaller, and have more a easily reached/compact distorted tone.
The Mesa F30, for example, sounds really nice, and has two EL84's in the power amp section...but EL84's produce only 11 Watts each of Class A/B power.
6L6's are generally better for about 22 Watts each, Class A/B power, and there are more "flavors" of output tubes to choose from.