The 990's manual has a good run-through of how to calibrate the system using a meter. When using the 990's internal test tones, you want to set the master volume to 0dB and adjust each channel to hit 75dB on the meter. Since you're starting out from scratch, leave each channel at 0 until you start adjusting.

The reference label is not sacred. For example, really efficient speakers might lead you to set each channel trim down between 5dB and 10dB - in that case, you could safely adjust all eight channels up by the same number of dB.