I have a Peavey XXX head that came stock with 6L6 power tubes, and changed them out for some JJ EL34's. The EL34's gave it a more aggressive, tight, in your face distortion, and the clean is more punchy. 6L6's are a bit warmer tone wise than EL34's. Personally I love the more aggressive tone of the EL34's. You should be able to find all the info on tubes you need at eurotubes like LOTS said. That's where I buy my tubes from. Bob is a great guy to deal with.