Monitor level
1. Buy a test CD with tones on it.
2. Buy an SPL meter (Radio Snack has one for not too much).
3. Set up the meter on a camera tripod, center its mic on your speaker about 3' [or 1M] away, and play back some pink noise through each side, one at a time (left, then right, or, if you're feeling adventurous, right, then left).
4. Adjust the volume control on each speaker to about 80-85 dB. As a bonus, you can the use the other tones on the CD to set subwoofer level, position the speakers and tweak whatever other parameters the speakers offer.
There is also the issue of gain staging, and there's no easy answer to that. Nothing in the path should be turned up all the way or down all the way. Aim for 0dB on the faders, for example. You are looking for a range where the circuits are neither loafing nor working too hard (there's a whole bunch of literature on this subject, starting with Gary Davis' Sound Reinforcement Handbook).
Enjoy.