NAD C700 no subwoofer output

Hi.

I set the crossover to 150Hz and enabled the subwoofer in the sound settings. However, there is no output signal at the "Sub Out" output (tested on 2 amplifiers, 3 cables and 2 sets of acoustics).
Turning on the subwoofer removes low frequencies from the main speakers, but does not send a signal to the "Sub out" output.
Am I doing something wrong, or is this a bug?