.Dark Sky & IE900 do share a similar sound, but I find the mids more forward on Dark Sky while mids on the IE900 are somewhat recessed. Dark Sky sounds more open & airier as well. Going from IE900 to Dark Sky it's almost as if a veil has been lifted. Bass on Dark Sky also seems to be more controlled while losing none of the impact, while bass on the IE900 to me sounds a little slower.
Every now and again I get super hyped about the IE900 then I read something like this and conclude it's probably not for me. Have you heard the Duo?