I've read quite a few posts about running the sub in parallel with the main speakers full range, but the NuForce amps are balanced; i.e. the negative terminal isn't at ground potential. This eliminates using the high level inputs.
There are most likely some ways around that. One way that works in many systems, although in a few cases (depending on the internal grounding configurations of the amp and the sub) it could conceivably result in a loud hum, is to connect the negative high level wire from the sub to the chassis of the DAC2, by putting it under a chassis screw. (I suspect that it would only be necessary to do that for the negative input terminal of one of the two input channels the sub provides, as its two negative input terminals are probably connected directly together within the sub. Also, I'll mention that I would have said to connect to the chassis of the amp, rather than the DAC2, if you were using a stereo amp rather than monoblocks). Each of the two positive high level wires from the sub would of course be connected to the + output of the amp for the corresponding channel.
Another approach, that should almost certainly work well, would be to purchase an RCA plug, perhaps at Radio Shack, and solder the negative high level wire from the sub to its ground sleeve connection, leaving the center pin of the plug unconnected. Then insert that plug into an unused RCA output connector of the DAC2 (an unused input connector will most likely work well also), thereby establishing a connection between the negative wire from the sub and the system's circuit/signal ground. (I would have said to insert the RCA plug into the unused RCA input connector of the amp if you were using a stereo amp rather than monoblocks). The positive wires from the sub would of course be connected to the amps in the same manner as for the previous approach.
Regards,
-- Al