Actually, slightly mixing of left and right channel while listening via headphones is greatly desired, and here's the explanation: If you are at a, lets say, classical music concert, front row, piano is on your 10o'clock, but you can hear it with your right ear, right? Same would be when you listen to that same music on your home HiFi, where that piano sound comes out just of your left speaker. You can still hear it with your right ear. Knowing that, music is often recorded in that manner, and it sounds best on HiFi, where you have full spatial orientation. So take that same music you listened on HiFi (and there it sounds great) and listen it on your headphones. It will sound unnatural, empty, you cant feel space between you and the instruments,coz it is just right ear, or just left ear. A while ago you could buy headphones which slightly mix and add delay between channels.
