I really havent looked closely at my 558's but they (and the 598) do have a more spacious sound than the 600/650. I believe that Sennheiser probably has installed some type of baffles to create some reflections / delays to achieve this. But there is very little about the 558/598 that resembles the 800.
I didn't think so either or else the 598s would be selling like hot cakes. I just thought they would have a scaled down version of the hd800 soundstage/spaciousness due to the "borrowed" technicalities of the HD800 (again see Steve Guttenberg's review).
Edited by Old Groucho - 11/20/13 at 5:20pm