what's sub bass? and what's the other pros in buying the e11?

hello, I've just got my ath-m50 and wondered if after the 40 hour burn-in I would get "punchier" bass?   If not, which portable amp would you suggest me to buy for it?   The m50's impedance is 38 and the volume is enough w/o an amp. My player is Sansa e250.   Thank you very much.