What you need is a pre-out on your integrate or a pair of them on the the pre-amplifier. I believe the Nad has amp & pre stages connected via a pre-out and a main-in on the rear of the unit. I don't know if it has a second pre-out though. You could always use y splitters, but I wouldn't recommend that.
If your sub has speaker level inputs, you can also connect it in parallel to the speaker outputs on your power amp/integrated amp with a second pair of speaker wires.