I'm just starting on a Super Reverb build and I see 3 jumpers on the board. I'm wondering what the purpose of these jumpers is. There's jumpers from R29 and R30, the channel mixer resistors. Are these there to allow access to preamp out, power amp in? What would this be used for ?There another jumper from R21 in the trem circuit, which honestly I don't understand. What is the purpose of this jumper?This isn't a show stopper, I just want to understand this layout, and I'm sure I'll have more questions as the build moves forward.