Most receivers have a test tone function. The correct method is to use a sound pressure meter (available at Radio Shack, among other sources) and adjust the channels (left, center, right, surround) for an equal level from the primary listening position. That's where the decibels come into play. Alternatively, one can 'ear-ball' it, but that is less accurate.

Other setting that may be involved are large/small speakers (directs where the bass frequencies are sent, larger speakers can better-handle lower frequencies), whether or not there's a subwoofer, distance from listening position to front speakers, distance from front speakers to surrounds, distance surrounds are spaced apart, and so forth.