Hook up a sub-woofer to Neumann KH-120s

Hi Everyone,



I'm an acoustic musician but newbie here in terms of recording / producing. So sorry of the following seems simplistic to professionals.



I just purchased a pair of Neumann KH 120 monitors. They sound great but they need some help in the lower frequency range. Neumann offers a matching sub which I'm sure sounds great but it's too expensive for me.



The boxes are connected to a Macbook Pro via a Presonus Audiobox22VSL



So my questions are :



a) Do other (cheaper) subs work?

b) The Presonus Audiobox22VSL has 2 outputs in the back, how do you connect the sub?



Thanks everyone :)

Hook up a sub-woofer to Neumann KH-120s