1+ oregonpapa, it has to do with our ears frequency response changing with volume and the volume at which the music was mixed at. If the music was mixed at a lower volume if you play it back at high volume it will be too bright. If it was mixed at high volume and you play it back at low levels it will sound dull with no bass. The "right" volume is somewhat different between systems. I like live rock recordings to sound and feel as if I am at the concert so I keep the very bottom end below 80 Hz boosted at a rate of 6 dB/oct. This gives me the sensation of being at a live concert at lower levels that won't hurt my ears.
Our ears accommodate to volume. The smallest muscle in your body called the stapedius tightens up on the stapes, the bone that vibrates your inner ear as the volume rises. It takes a few minutes for this to happen. It is therefore always important to warm your ears up. I start at about 80 dB and add 2 db or so every two or three minutes until I get up to 95 dB. This gives the stapedius time to tighten up and protect your inner ear from noise induced damage. This is also why impulse noise like gun shots is the worst to cause hearing damage as your ears have not had time to warm up.
