Trying to understand optimal method of volume control with separates

I recently got a vtv eval-1 stereo amp to go with a topping d90/a90 dac/amp stack which is intended originally for headphones but doubles as a dac/preamp going out to the eval-1.  The d90 dac has volume control with a remote which is great for relaxing on the couch since the a90 amp/preamp only has a knob you can turn by hand but no remote.  I could be imagining things or recovering from an un-diagnosed ear infection but I could almost swear it sounds better having the dac at 100% volume or alternately having the volume knob on the preamp turned up higher with the dac lower.  Is there some optimal combo of preamp voltage and dac volume that fills in the music better somehow? 

I assumed that one multiplies the other and the only thing that matters is the total that comes out into the speaker amp but for some reason it sounds more full/even the other way.  The only problem is when I have the dac at 100% there's almost no room to adjust the volume knob without getting way too loud.  Having the preamp volume knob permanently up higher and setting the dac lower also seems to do the trick but I'm curious if I'm just insane or if there's something to it.