you definetly can hurt your hearing with inear plugs -.- , im 20 and have already damaged ears from those.... when its quite i hear a high pitched noise..... simmiler to the Static from tv speakers turned up too load with no sound playing.
my acutal hearing seems totaly fine tho, i can still hear all freqencys from low 35s 45s upto 15000 - 17000 hz.... and i can hear very faint enviromental sounds that most people miss/dont pick up on.
High frequency sounds of 2-4,000 Hz are the most damaging.
and i can definetly tell between good and back speakers/head phones/low quality files
below 320bit rate.
back on topic, i have only ever had very cheap headphones die from being too load... my $80 headphones always outlasted the cable ... witch both broke off after about 2 years of abuse ^^. one at the headphones and the other at the plug. the speakers themselves were totaly fine.
as for maxing out the speakers volume. you can run the Z-5500s at there max volume 90% to 100% (Not boost mode) for hours and not damage the speakers (using high quality audio files) the system peaks upto 115 to 125 decibels. (very loud
)
here is a quick decibel comparison
http://www.gcaudio.com/resources/howtos/loudness.html
(make sure you have ACTIVE cooling on the amp, it gets hot.) 505 /1010 peak watts has to go somewhere. keeps u warm in winter
to answer black panthers question about the safe limit on speakers + amps.
i belive they use stronger amps then the speakers can handle for one main reason. you dont want to stress the amp 100%. if the amp is too strong for the speakers then when you crank the volume just before distortion you will only be using (Example, NOT TRUE FIGURES) 75 - 85% of the total power of the amp so it wont overheat and die sooner then it should.