Interesting, the Flat4 are even more dynamic than the IE800? I believe the Senns have very good dynamics, and as a result, impressive depth. Don't remember where I got that impression from, but I thought the Flat4 sound rather 2D with little depth, which usually goes hand in hand with limited dynamics. May have been wrong about them all along.
The Flat4 have what I tend to call "inner depth." Compared to the IE800 they sound a lot more like they're inside your head, between your ears. For me however this isn't the same as flat. The RedGiant A03 for instance sounds like it's outside your head, but it lacks depth, so the effect is like watching a big movie screen.
Maybe I need to spend more time with them, and maybe fit is playing a big part, but so far I don't think the FXZ200 and IE800 sound remotely like each other.