It could be as simple as a loose speaker wire, or it could be as serious as a blown tweeter. Hard to tell from the description, should take it to your installer and have them diagnose it.

(Side note: If this is empeg-specific, it could also be because you are boosting certain frequencies above the 0db digital threshold. What is the volume setting on your empeg and what does your EQ curve look like?)

Something I notice on my own system is that one of my four channels will distort occasionally because the potentiometers on my amplifier are cheesy. If I turn one of the potentiometers back and forth (I think it's the "gain" pot for that channel), the distortion goes away for a while. I think it's just dirty contacts on the pot and I have to sweep the contacts occasionally to keep them working right.