I own the original HE400i headphones. Imaging, transient response, treble, soundstage, etc. are all stellar. Just go read the RTings review. But, as you can see in the FR graph, bass isn't so great. It's well defined, but there is no punch/thunder. This is a characteristic of most open headphones, but these are worse than most.
And yes, you need a headphone amp to get anything out of these. Even an O2 will do the job, but your phone won't.
And yes, you need a headphone amp to get anything out of these. Even an O2 will do the job, but your phone won't.