Typically sub bass is below 80hz while mid bass sits around 80-150hz. Most headphones will easily reproduce mid bass, while it is more rare to find a headphone that will accurately and flately reproduce the <80hz frequencies.
Headphone specs are mostly useless. The Akg k702's specs say it goes down to 10hz. Having used these headphones for mixing for a couple of years I can say although they reproduce the entire spectrum with great detail, the bass rolls off below 50hz, meaning you don't get a complete feel for the body of a bassline as well as on a full range monitoring system. This deficiency can be alleviated through practice and listening to music through the headphone so you know how each headphone sounds and feels.
In my experience closed headphones and iems are better for bass mixing due to the sealed design, but open is much better for judging the rest of range. (Of course, this is a generalisation)
Basically, no single headphone or speaker is absolutely perfect. Use whichever headphone or speakers you're most comfortable with to do the initial mix, then listen to it over and over again on different headphones, earbuds and speakers and adjust accordingly.
That being said, I do like using my etymotic ER-4S, phillips x2 and AKG k702 for most mixing because of their relatively even response, but they're just my preferences
