Best thing to do would be to find a store where they have demos and listen for yourself.
If you can find a used pair of WS99 they are better than the M50. The Head-Fi classifieds are a good place to check.
Sub bass is 20hz-60hz, mid bass is 80-160hz, upper bass is 160-300hz. Some headphones only have midbass and upper bass (like the older Beats) with no sub bass. If it has sub bass and mid bass that is a good thing. You don't want upper bass to be emphasized, or else the mids will sound muddy.