If you're pushing them hard, I'd limit amplifier output below 40Hz with a subsonic filter, if possible. Alternatively, just listen for bottoming out, which sounds like you'd expect, sharp impact sounds. It sounds sort of like detonation in an engine, an unusual clacking sound. Also, from about 1kHz up, the system can't handle more than about half that much power. But this probably will be no problem because of the distribution of sound in the spectrum. Watch for the light bulbs glowing, and that will tell you you're getting up there.