Some amps have it so you can adjust the bass and everything right on the amp.
As with the the other thing do you always want your subs come on or do you want them on a switch to turn them on and off.
Every amp these days do, but if you're running of the stock radio, the only way to turn the bass up on the subs is to turn up the bass on the deck. Which will bring the bass up on all of the speakers. Unless you have a remote bass knob.