Hey there, Neil - the 750 has moved on to a new home and the cats are well (daughter got a cat toy from her grandparents for Christmas, so the cats have some extra entertainment these days).

Audio only sources (CD, tape, aux) of course have no video source associated with them, but you can also have video sources with an audio input from a source (optical, coaxial, or analog) and no video source connected - the 970's monitor output won't give you anything because there will be no signal for it to use, but since you'll be using the VGA input at that point it won't matter.

What sources will you be using that offer VGA outputs? Will you have a single video source tied to that VGA input, or will you also have other sources that will use component or DVI/HDMI?