When two like signals are combined, the summing effect produces a signal that is hotter. Most analog consoles are designed to cut the signal in mono mode so that the voltage of stereo and mono monitoring modes are approximately equal.The issue surrounding level has to do with the correlation of the signals on the left and right channels. Material that is 100% correlated left-to-right (a vocal that is panned dead-center, for example) will theoretically build by +6 dB when left is summed with right (L+R). Uncorrelated stereo sources (like far-field-recorded classical recordings), only build L+R by +3 dB. The choice for correct mono summing, then, depends on the left/right correlation of the material. Pro Control uses the equal-power (-3 dB) compensation approach instead of the equal voltage (-6 dB) compensation.