To those concerned about their listening / SPL levels -
80-90 dB for prolonged periods WILL damage your ears (1-2 hours).
Many speaker systems, especially those of high quality, can reach well over 100 dB (say into the 110 dB range). Given the power of the O2 and the HE-400s, I estimated awhile ago that about 115 dB is possible (the excess is 'headroom' - meaning I don't want to listen that loud, I just want compensation available for weaker source material). The power delivery of the amp is not linear, and it has protection built in, so realistically I'm guessing that those who listen between 9 O'Clock and 12 O'clock are within a safe range (that is about where I can be without experiencing fatigue rapidly). Max O2 power is waaay too much for typical recordings.
To understand what 80dB sounds like, get in your typical car and drive down the highway. Even with the windows up, typical cars are putting approximately 70-74 dB to your ears. Now keep turning up the radio until you have totally drowned out the noise. Now, pull over completely and shut off the car. Wait a few moments. Turn it back on, and listen. The sound glaring at you is probably approximately 80 dB or so (or 'twice as loud' as 70 dB). Given the noisy environment of a car, it is a great place to damage your hearing!
In a quiet living room, the ambient noise level is typically going to be in the 30-40 dB range (I believe). Therefore, a speaker system of any type should easily overcome the noise, and provide good listening at a safe volume level. Now fill that room with people and alcohol, and bear witness to speaker systems reaching peak volumes in-home - you are back into the 80-100 dB range depending on how good the party is. : )
Remember that most headphones require 1mW to reach 90 dB or so, which is already approaching ear-splitting. That is, 1/1000 of a watt. Many stand-up or car speakers required only 1 full watt to reach the same volume, and only 50-100 to reach extreme volumes.
Some of you that are most concerned about your ears would be wise to pass on the 10 Watt Super-Amps frequently claimed to be 'required' for headphones such as ours. They are not. The power is typically an utter waste, unless you spend all your time listening to Classical or High-Dynamic-Range recordings. Some of which, btw, being uncomfortable since they hit you with peaks that can be extremely jarring.