IMO you don't EVER use headphones to mix/master with. Always use a nice pair of Active or Passive Speakers. You can get a sexy pair for around £200-300.
The problem with headphones is that you cannot tell the amount of low end is in the mix. The levels won't work in the speakers because after 10 minutes of using headphones, you automatically push sounds 'louder'.
I'm studying Music Tech as a degree and have been recording for 5 years now, from my experience this is the case and all of my lecturers would agree so.
Hope this negative message helps.
