the 2101 has 2 inputs on it, and a switch to select which input you
want to use, A/B it has a seperate sub volume control, a Q factor
selection knob, and a sub frequency selection knob, plus it has 5
bands of EQ adjustment i believe 250, 500, 1, 4, and 10Hz, the sub
output is selectable for what freq you want to adjust from anywhere
form 25 to 100 HZ. the Q factor means that you can select how wide of
a response curve you want to select for the sub output, it is very
precise in it's adjustment ability.