The NAD volume control should work. Volume is not supposed to be controlled by the DVD player or by the amplifiers. A DVD player provides a standard audio output level which varies depending only on the relative sound levels in a movie DVD or music CD. (With a computer, you should set its audio output to a fixed amplification level. 100% usually is appropriate.) The amps are intended to provide a fixed amount of amplification for the signals coming from the preamp. The volume control knob (or up/down button on the remote) specifies how much the audio signal from the DVD player is attenuated or amplified within the preamp.
The first thing you need to do is reset the NAD to factory settings. That way you know exactly what you're working with. There's no way to know what configuration your father might have been using. See pages 17 and 20 of the Owner's Manual.
Please provide the exact model numbers of your DVD player, of your computer and its audio/video player software, and of your amplifiers so more detailed help can be provided.
Marantz SR7009/7.1.4/FH+TM/DefTech PM1000/LCR+TM amped