Couldn't find an answer for this exactly, so here goes: I bought a pedal, an Electro-Harmonix English Muff'n and the description is as follows: a glovered 12AY7 cascading into a 12AX7, to produce true Brit-valve sounds that are full of richness and natural saturation. And being a guitar effect, PLUS the fact that it's supposed to emulate Brit-valve sounds, I expect the low-end response to be thin and lacking. Here's the tube questions: 1) What does the "12AY7 cascading into a 12AX7" mean? Is that that a signal passes through a lower gain tube, and then a higher gain tube? What would this accomplish? 2) I am told that the 12AY7 can be replaced with higher or lower gain tubes like a 5751, 12AT7, or 12AV7 and the 12AX7 can be replaced with a 5751, 12AT7 or 12AY7 and should work right. What would putting a higher gain tube in front of a lower gain tube accomplish? What about tubes of the same spec? 3) Are there any particular brands of tube that offer mellow a particularly warm sound with good bass response, or is it all a crapshoot? 4) This is gonna cost me a bit of money to find what I like, ain't it?