The switch is to take care of the voltage problem. It's really not that big a deal. The only thing that would happen if you didn't have the switch (and your HU volume was turned all the way down) is that your empeg would be a bit softer than it should be (i.e. quiet). That's what the switch solves.

As for where to mount it... If you can't fit it in the armrest, try the golve compartment. I'm pretty positive it'd fit in there with plenty of room to spare.