So I know practically nothing about vacuum tubes. I see some of you guys replacing the EL34 power tubes in your Marshalls for some of these power tubes,...6ca7EH and KT77. I'm wondering what these tubes will do, for the amps your putting them in, that EL34s do not? Maybe that's not the best way to put it. What kind of tones do these tubes create in your Marshalls that make these tubes desirable? I have a SC20 that I've replaced the stock EL34s with a matched set of Mullard New Production EL34s and to me, my amps sounds a bit more dynamic,...for lack of a better word. So what would these other power tubes do for my SC20? Thanks!