NAD amp doesn't need to be on use the Q1 with the PC for headphone use. It also doesn't need to be on to receive audio from the Humax box. That's the point of the stackable RCA cable.
PC use does bring up something that hasn't been mentioned yet, and that's an output from the Q1 to the NAD for speaker use with the PC.
The Q1 has a dual function port for line in and out. I'm not totally clear on how this works, but as I understand it, you have to flip the charge switch to change between input and output. From what I understand, to use the Q1 as a USB DAC, the charge switch is set to off. This will output audio from the PC to headphones connected to the Q1 and also the 'line out', which will be connected to the NAD.
Setting the switch to charge will disable the USB DAC and 'line in' will be enabled, so any headphones connected to the Q1 will receive audio coming from the Humax box.