iPhones speakers for listing to music is not really good.

You need to disable all music profiles e.g. Bast-Boost and so on, so that you have a decend sound quality. Listing with the provided earphones you need these profiles.
And... WHY is it always the iPhone? Best phone ever for calling quality was my 9500 xD and listing to music.... the Nokia 7710... it has real subwoofer built in