Hey, long time listener to the forums, first time caller. I've been reading here on and off since 2005.
I currently have both the E4C and the SE530. I picked up the SE530 about a week ago, and I've been listening to them practically non stop.
Here's a quick summary of the differences in sound quality
In relation to the E4C, the SE530 has dramatically more bass whilst still maintaining accuracy. The 530's still have accurate, tight bass but with a very pronounced presence. You can practically feel the drums and lower end, and it makes electronic music really shine (I was using "CSS - Music is my Hot Hot Sex" as a reference where it REALLY shines)
The mids in the SE530 aren't as bright as the E4C. The SE530 by no means is lacking in mids, it just doesnt have the acerbic quality that the E4C's display (especially during guitar distortion). The mids in the 530's actually feel more distinguishable. I find it easier to determine strumming patterns with electric guitar distortion on in the 530's.
Honestly, the SE530's earphones are the closest sound you can get to live performances. I just saw CSS in concert this weekend and these phones REALLY sounded close to what the band intended their music to sound like.
Overall, the SE530 really outperforms the E4C's, and I'd consider it a good upgrade. I tried the SE310's before, and felt that they were just E4C's with boosted, but less accurate bass and tuned down mids.