What is the proper loudness for listening?

I have a big space and the system doesn't really come alive much under 75db.

Having read somewhere that listening above this level on a regular basis can lead to hearing loss, I don't exceed it.  I have no wish to compound the damage already done by guitar amps. There's no sense of "missing out" by not cranking it louder, though. 

I do automatically turn it down when my wife wants to listen. It only makes sense to  do whatever I can to ensure she enjoys the system.  

@ditusa 

Well, OK. When I worked as a baker in Eugene, our sole means of ventilation was  a small fan mounted in 3X3'  window. Standing over a mixer churning flour dust into the air all day, I was concerned about my respiratory health. I called OSHA  The first  thing the guy asked was "can you see across the room". With this question alone, he made it abundantly clear that the regulations (no doubt written by industry lobbyists and rubber-stamped by elected representatives on their payrolls) were a joke. As a consequence, I don't have a lot of faith in govt. guidelines.