The NU3000 can be Bi-Amped within the amplifier itself which allows the highs and mids to go to one output while the lows go to another, all of which is crossed-over at 100 Hz.

1.  On the back of the amplifier, switch the MODE to Mono
2.  Set the CROSSOVER switch to the right, which set the internal crossover
3.  Connect your source to input A
4.  Connect your mains speaker to output A
5.  Connect your sub to output B

The amplifier will now be able to provide signal to both your main speaker and your subwoofer with the appropriate frequency ranges running to each.