My audiologist told me sometimes sibilance has to do with the way the sound travels down your ear canal and hits the drum. I noticed that with my westone 3s I have fewer problems with sibilance if I use the long comply tips or the triple flanges rather than shorter tips.
Shure SE 535 Sharp "S" Sound - Page 2
Gear mentioned in this thread:
The more I listen to different audio equipment, the more I'm convinced its a problem with my ears. On the up-side, I recently got my FiiO E17. To conclude, I think the DAC in it helps with the sibilance a lot, even more so than EQ'ing the sound. Instead of being unbearable, it's MUCH more tame and just there. It isn't a complete solution, but its satisfactory to me. Thanks for the outstanding replies to this thread everyone!