Think of sub bass as the deepest, lowest rumbles in bass, and mid bass is a bit higher in the spectrum, (typical in drum and bass music).
I'll try and see if I can find a good comparison... might not be the best...
Sub bass:
Vibrasphere: Erosion
Mid Bass:
Pendulum: Plastic World
Pay attention to the bass, you'll notice that the Pendulum track is a bit higher in pitch in general.
Mids:
Usually where vocals are. With bass and treble heavy phones, mids take a step back (obviously), so vocals sound distant, whereas bass and treble are in your face. With a headphone like the HD598, vocals tend to be more upfront compared to the bass and treble.
That's not all the factors to the sound. Lots of songs have their own balances, which is why sometimes a balanced headphone like the Q701 can sound bass heavy, and some songs can have distant vocals no matter what headphone you use.
If you don't know highs/treble, well then, quit head-fi, lol.
Oh yes, 'Pendulum - Salt in the Wounds' does both sub bass and mid bass pretty close to one another. You can easily hear the range changed between sub and mid bass for some of the effects.
I'm not an expert on saying when sub bass ends and mid bass starts, but I can usually hear the diff, because the D7000 doesn't really do much to mid bass (it's more or less in line with the mids). It's emphasized quite obviously in sub bass.
I guess the difference would be easily told apart with the XB700 and XB500, which excel in differing areas. the XB700 is really sub bass heavy, and the XB500 is mid bass heavy, though it can go quite low itself, just not as much as the Xb700.