What volume level should I set my D1 and/or speakers to for the best results?

There's not really a right or wrong when it comes to volume settings.  I'd recommend setting it up whichever way is more convenient, and dialing in a combination of settings to give you the smoothest ramp-up.  If you intend to use the D1's volume as your main control for example, try this: set the speakers to zero volume, and the D1 to max volume.  Then with music playing, adjust the speakers up until the volume is as loud as you would want to listen to it.  Then simply adjust from the your D1 to mood.