A volume setting of 0.0dB for something with a "relative" volume scale (including all of Outlaw's processors and probably Anthem's AVMs) is typically the point at which the system is set for "reference" operation during calibration. On the Model 990 and other Outlaw processors, the internal test tones are meant to play at 75dB when the volume is set to 0dB. The AVM20 is probably similar. The trick is to take some caution during calibration - set the unit to an unused or inactive input so nothing is playing, then set the volume to 0dB, and then run the internal test tones and set each channel's trim so that it measures at 75dB in the listening position.

Once you've calibrated, you may only use that volume setting on rare occasions. Some folks do it all the time, and so folks never get that high again. We watch TV here at around -35dB to -25dB during the day, depending on the channel (some HD channels aren't as loud). DVD and Blu-ray watching tends to involve a slightly higher volume setting, but unless I'm home by myself it's almost unheard of for us to get higher than -20dB or maybe -15dB.