I've got the 400i and really love them. But, I just can't shake the idea that I like the 500s a TAD more. They both retail for $499, so I could probably do a straight trade with someone, but can't make up my mind.
The 500s I heard were heavily modded, which I'm sure impacts things.
Based on memory of the 500s, the bass was a little boomy. I prefer the 400is tighter bass. The 500s also lacked a little edge/bite on guitars, especially acoustics....