Time for a real audio post. It's been mostly diy up til now.
Gain structure. It's really important. A post over on djforums got me started on this. One of the posters disagreed with the method, different strokes and all that. I know it works, and works fabulously well. The two most definitive places to find the info from the real pros are at http://www.rane.com/note135.html, and http://www.vizear.com/downloads.htm. Both these places sell a nice little doodad to perform the test. The clipcop is much more sophisticated. The technique is based on information from Syn Aud Con; who provide training and seminars for audio professionals. The vizear link is the actual articles from Live Sound International.
You don't really need their doodads, but they do make it easier. All you really have to have is a piezo and a rms voltmeter. The basic idea is to find where every piece of equipment in your gear clips. Then you set all of it so it clips at the same time. If you see a red light on your board, you know the entire chain is clipping. It takes quite a while to set up. I think I spent an afternoon, with a 12 channel board, outboard dsp, compressors, crossover and the amps.
But when it was done, indisputably the system sounded better. Plus I was much more confident in it. I set the main board so when the level led's bumped yellow, there was still about 5db left in the system. That way if it wasn't me at the board, all I had to tell them was you NEVER let the red light up. And if they did, (which is just about inevitable), there was enough head room available to handle it, both in the electronics and the speakers.
The other big plus is when you're done, you'll know your system's capabilities inside and out. I never ran limiters, it instilled that much confidence to me in the system.
Oh, and everything at the rane link is worth reading. I mean everything.