I posted this on another thread for how I measure SPL for my IEM. I'm not sure how accurate it is. Any comments are welcome
http://www.head-fi.org/forums/showpo...&postcount=102
In the same thread, Skylab posted his method of measuring full sized cans.
http://www.head-fi.org/forums/showpo...5&postcount=42
You can't tell if a position on the amp's volume dial is too loud unless you have measured the amp/headphone combination.