I do not think there should be any problem using high level inputs on subs the normal way with Ncore, but to be certain I would send Hypex an e-mail asking about it. The output stage of the Ncore amps are essentially ground referenced: the black speaker terminal is ground, and I do not recall there being any potential above ground.
This was a problem with the ICEPower amps, as they sat around 40 volts away from G (but it could be dealt with as well by adding some caps), and balanced output amps need to the sub high level cable to hooked up to + and the amp G (not -), but Ncore is single ended output.
I am not sure, so ask Hypex, but I am now curious and will go and measure the Ncore now...