Dont ditch the E6 yet. It is a great gem for the money, increases volume a crapton, can give you treble, or bass.
You said you want bass? Well the E6 can give you bass for sure, if you really want an amp, need more bass, and can only spend 20ish$, the E6 is fine. Now, dont get me wrong, the ZO or ZO2 will give you more bass than the E6, but in the right EQ mode for the E6 (I cant remember what mode it is, I jsut know it's the red one -.- ), now, for some headphones, the bass is quite overshadowing for some music, and on naturally bright headphones like my V6s, the bass seems almost out of place and weird, but on headphones like XB500s, it feels like they are supposed to be part of the XB500s in the first place.
So what headphones do you have? Amps and headphones should be paired, not mixed and matched.