I have a Marshall TSL - 100 and i am thinking about swapping the EL34 tubes for 6L6. How big a deal is to do this and what brand of 6L6 would you recommend? Is there much difference in sound between brands? I play a combination of progressive/heavy/thrash metal.
6L6s require less heater current than EL34s, so the mains transformer will be fine. The output transformer's primary Z won't be optimal, but should be fine - plenty of amps out there fudge the PT's primary to suit, it's really not as critical as many people would have you believe (valve amps are very forgiving). However, just popping in a set of 6L6s in an amp set up for EL34s is asking for trouble, you'll at least want to set the amp's bias cold as it'll go and take it from there.
You'll get more bass and less crunch in the mids with 6L6s rather than EL34s.
For the styles you list, I'd go with 6550s - bigger/tighter bass, all the crunch that EL34s have and a more agressive overall tonality (plus more headroom). Every amp I've ever put 6550s in has improved the tone. You'll need more bias voltage, so the bias supply will need tweaked, but thats nothing a tech can't do in an hour's worth of bench time.