a good tip: if you take off your headphones and hold them about 2 feet infront of you, and you can hear every part of the music in a fairly quiet place, then it probably is too loud
This tip isn't universal - I can hear my RS-1 playing at a VERY soft level from across the room, simply because they leak like crazy. But I can have my IEMs at ear-destroying levels and not hear them hanging around my neck.
So, I've got a pair of M-80s, and was wondering the following things:
1. Does playing on maximum volume harm the cans, long-term?
Define "maximum volume" in a more concrete way. Maximum that the headphones can take? Or that you can take? etc.
The "output level" on any device is irrelevant for this discussion because gain/etc are unknowns and without measurements of the device output it's not actually possible to say that "80% output is 80% of the device's capability." It also depends on the sensitivity and impedance interaction of the headphone to the amplifier as to the overall acoustic output as well. It's a somewhat variable discussion.
See here:
http://www.audiokarma.org/forums/showthread.php?t=58829
2. Are my ears safe if I use this pair on max, or should I probably reduce the volume?
Per OSHA and CCOHS, 85 dB for 8 hours a day is the entry to NIHL - 85 dB is pretty loud though (then again, I've seen a lot of young people these days who aren't even phased by 85 dB, which is sad; pain exists for a reason). If you don't have headaches/pain/etc associated with listening that's a positive, if you DO though, that's too loud. Your ears are not safe with any production headphone at it's maximum output level (they are ALL capable of destroying your hearing in an afternoon).
3. What is the best volume for audio quality (for good sound / bass, etc.) ?
After a certain point the Acoustic Reflex will kick and decrease volume and DNR ability (ever heard a NASCAR or similar and then everything else sounds "dull" for a while?) - so yes playing too loud will reduce fidelity. I forget where the exact level for this is, it's either 90 or 100 dB.
Ultimately if you aren't experiencing any pain or fatigue or ear-ringing after listening, I doubt if you're causing damage, but don't take my word for it. If you're really worried, try measuring the output of the headphones with an SPL meter (which is the only instrument able to really do what you need here) - anything over 85 dB is bad for any sort of long-term use, the 60-70 dB range is very comfortable though.