Well, from Ohm's law, V=IR, and P=VI. I'm assuming you want maximum power (P) from the sub, so solve for P and you get P=I^2*R, or Power is equal current squared times resistance (measured in ohms). So what you're doing when you change this resistance is varying the effective power that gets to the sub.
In general less power will yield better sound quality. So less resistance will yield better sound quality. This of course makes the system lose the punch. Basically, start with the resistance pretty low, and turn it up until it doesn't sound good anymore.