Zero is considered "reference" level, but I don't think there is any kind of industry standard to appoint it as such. Usually, by the time you get to "0dB" it's pretty damn loud.

I don't think I'd want to try to find the maximum level on a receiver. My ears might start bleeding before I even get close.

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"Nothin' up my sleeve. . ." --Bullwinkle J. Moose