To those who seek for a portable headphone which also have good low end, and don't wish to go for IEMs (many of which have very good low end), I'd warmly recommend the Beyerdynamic DT-1350. You won't find many headphones with a low end like this one has, especially at this compact size - it's very small and folds (Grado style) into a carrying bag for travel. I use them everyday, wish they were more popular in the consumer headphone market. Try to audition them if you can.
These are 80 ohms but are driven nicely from a laptop. They do improve with amplification, it adds some more 'texture' to the overall spectrum and provides a more balanced sound.
Speaking of the sound signature, they have a gentle emphasis at the low mids, say 400Hz, but it doesn't much except some warmth and 'weight'. For mastering through headphones you'll always have to know the weaknesses of your headphone and compensate for them. There's no perfect headphone for this matter.
Here's a link:
http://europe.beyerdynamic.com/shop/hah/headphones-and-headsets/studio-and-stage/studio-headphones/dt-1350.html#By the way, these are ON-EAR headphones, not over the ear like the 7506's are.