Ok, I didn't see anyone else touch on hertz-related hearing loss, so I'll proceed.
It all depends on what hertz (Hz) you are really pumping. At 20Hz (the practical low for a sound system - few people can actually hear below 15Hz or so), very little damage is done to your hearing because it is so low.
At 20,000Hz (20 MegaHertz), if you turn it up loud, you will get hearing loss MUCH quicker. 20MHz is the practical high for a system, few people can hear above 20 or 25 MHz.
Where do you really want to blast your system? The bass, of course, from 20Hz to about 100Hz. The higher octaves you don't want to blast because higher is treble - that sucks.
Keep your tweeters down, blast the bass, and keep the midrange comfortable - you'll be fine.
-- Mike (eh, what did you say?
I hope I didn't make that too confusing...