I'm no expert, but if an amp comes with EL34s chances are it will need BIASing for 6L6s. I know there are a few amps that can use both (the Laney GH50L for one) but they are few and fare between. Some amps do have a trimpot installed so all you have to go is find the right voltage and turn the trimpot to it, others are a little more complex and may require soldering of resistors.
Even a 20watt valve amp is hard to push at reasonable volumes, i've got a 1/2watt valve amp and i can't turn that past 2 without it becoming over-loud.
You might want to ask Wez about metal sounding amps. I know he likes loud (as in 100watt +, more head room) because he doesn't like the sound of power amps breaking up...all preference.