15 amp vs 20 amp

Hmmm. Good question. Ideally, a separate direct line, or if that is more than you can do, along with the amp. But, I have never experimented with adding a sub to the amp circuit. The sub is an amp… you don’t want to constrict flow. I am sure the former is best. Combining multi-Le amps will not be ideal. 

Best practice is to run two direct lines, one for the power amp without a power conditioner and one for the power conditioner with the rest of the components plugged in to it. I can't think of any reason you would want a power conditioner rated for 20amps. I don't know of any amps that pull that much power. Even if there are, I assume you don't have one. I can't see any benefit... high quality is what you want in a power conditioner... like Shunyata, or IsoTek.