Your ears might need a break, and you might also be listening really loudly - I know when I compare or switch between headphones sometimes, I end up cranking the volume more than I should. Also, if I'm listening too much (or at too high a volume) some days everything just sounds bright, other days, things sound dark, and other days things sound thin.
Also, they are quite different than the other headphones you mention, sometimes Senns sound crazy dark and dull to me if I've been cranking the Grados for a while, and the Grados can definitely sound bright if you've been listening to something else.
It also depends on the music, of course.
I apologize if I'm just stating the obvious - but if you've been switching volume, or what you're listening to (or both), or just listening a lot and your ears need a break, lots of wild effects can creep in.
BTW, I've got my RS-1s turned down to a nice moderate level and have been digging on them for the past 30 minutes or so, even though they are known to be perhaps a bit bright as well