It's been awhile since I listened to the HD280's so I'm going by memory here. The two are very different. THe HD280's are a somewhat analytical phone, treble can be a bit on the bright, sibilant side but not too bad. Mids are a bit on the dry side with maybe a touch of warmth to em. The bass is the big source of contention with these. The midbass is lacking so they come across as somewhat bass shy but the extension is phenomonal, I've yet to hear a headphone that does the lowest registers the way the HD280's do. Overall they're not a bad headphone, a decent under $100 closed headphone that will give you lots of isolation but they look dorky and they can be kind of tight and hot.
I've only had the E4c's a couple of days so I'm just starting to get familiar with them but in my opinion the E4c's are in a completely different league than the HD280's. They offer a very balanced, detailed presentation that is just on the warm side. Highs are quite detailed and extended without being overdone, mids maybe just a tad forward and IMO the bass is quite good. When you get a proper seal they have plenty of volume, quite punchy and extended with great detail and texture. Detail and texture are not exclusive to the bass either this is a nice trait they have been showing the past couple of days. Being an IEM the soundstage is on the small side but is quite defined and you get a good sense of the recording space. Overall the E4c's offer a seamless presentation where no one freq. band dominates. When a recording has is bass heavy it shows, if a recording is on the bright side it shows but they always seem to be listenable.