I was just getting ready to ask if you meant EL34, I just got off from the Peavey website and saw that the XXX was switchable from 6L6 to EL34, the EL34 is probably what you want to use if you are trying to Marshallize your sound, the EL34 was used in the classic Marshall amps and it's been a pretty common swap in tube amps to try to capture more of that type of sound, the EL34 distorts a little sooner than the 6L6 but has a softer, warmer type of sound, anytime you are changing power tubes the bias should be checked/adjusted, I don't know if the XXX has an adjustable bias since I can't find a schematic for it, but a good tech will know how to set the bias if it needs it, as far as pre-amp tubes go I like EH and Groove Tubes, they both seem to be cleaner sounding than say Sovteks or alot of the china tubes that alot of manufacturers put in their amps, I don't know what came in your amp, but Peavey is very China friendly and if the tubes are marked with the Peavey brand name lets just say that they probably weren't made in Mississippi. Being new to tube amps it's easy to get carried away with cranking up the channel volume and turning down the master, when you do that you are driving the pre amp to distort and thats mostly what you are hearing, power tube distortion is softer, warmer and doesn't have that grainy, grinding type of fuzz box sound that pre amp distortion has, if you have a place where you can really crank it up, set your master on about 8 or 9 and then start turning up the channel volume, you'll hear alot smoother distortion, well keep playing with the knobs and have fun!.....................