Well I decided to tear in to it myself which scared me a little bit as I have had bad luck with tiny wires in my life. Here are my findings and some observations.
The plugs are not the same. They are very similar the same style even but the 280s have a larger plug. The four pins are a heavier gauge and are spaced further apart than the 555s.
The HD280s have a very slick system for removing/replacing the cable. Each cup has a separate back plate and would be very very easy to do a dual entry mod. This would be weird if I was actually using them for DJ use but I think it would be nice for the way I (and I imagine most of us) use them.
I would still be interested in finding a source for the 4 pin plugs of either style because I can't decided if I want to drill holes or not, at least not in the 280s.
