I am not a crook's head
Since: Mar 14, 2003
Ideally, you should purchase actual monitors for mixing.
Mixing in headphones can pose problems since most headphones have to do weird things to the frequency spectrum for them to sound natural to your ears since they use small drivers that are so close to your ears. You're usually not getting an accurate picture of the sonic spectrum through headphones.
That said, I still do most of my mixing in headphones because my phones are much more detailed and accurate than my current speakers. You want a good reference-quality set of phones for mixing.
If you were just using them for tracking, sound quality wouldn't matter much, just minimal leakage. An average $20-$50 pair of AKG, Nady, or whatever would do.
But for mixing, you want an accurate, detailed set of phones. I use the Sennheiser HD-240 Pro phones and they're terrific except for a weak mid-low frequency response that'll cause you to make boomy mixes if you're not careful. Listening to a 24-bit mix in these phones is amazing though. The clarity and dynamic depth really shines through.