I'm hoping to resurrect this thread after being dormant for 13 years. I too am confused by output voltage from different sources and how it relates to volume.
I have a Parasound D/AC-1600 which states it has an output voltage of 6.0V through the balanced output. My Theta Pro Gen Va has a stated output of 7.0V through the balanced outputs. You'd think the volume of the Theta would be "louder" than the Parasound, right? Well, it's not.
Through my Pass XP10 inputs 1 and 2 (both balanced inputs) when you switch back and forth, the Parasound is about 6 clicks louder on the volume knob (6 dB?). I've had other DACs hooked to the balanced inputs that were lower output voltage, and they (as expected) were not as loud as the Parasound. Could the specs be wrong on the Parasound? Any other explanations?
Sidebar: how cool would it be if you could preset the volume on your preamp to different levels by input so you could volume match different sources - does that exist?