After doing some extensive auditioning and having recently purchased the HD800's I would only recommend someone to buy them after being able to compare them with other headphones. They aren't perfect, but at the very highend what you end up with are slight differences between the cans (hotter treble, wider soundstage, boomier bass) with each one being a compromise of sorts. I pick up the sibilance as well but I do not think it is being added or too overly exaggerated. When I compared to the D7000, I could easily hear the same sibilance (though it was just a a bit less pronounced). When I went back to the HD-650's I could hear it as well but it just did not stand out like on the HD800's. I do notice though, that the amount of sibilance can vary from song to song even on the same album. I have also listened to audio without any sibilance. For example, I just used them last night to do a run through of the latest episode of the Sam and Max computer game. The game lasted around 3 hours and is largely dialogue driven but I did not hear any sibilance whatsoever.
Give some other cans a try and see if you can pick out the same deficiencies. Definitely though, when I switch back to the HD650's, I can really tell a difference in the quality. Kinda sneaks up on you.
EDIT: I am very surprised though that you find the bass overblown. I feel that the bass is just a tad less than what it should be. But it is absolutely anemic in comparison to say the D7000's. I put in a Clash album and the Denon's were booming constantly throughout the entire song. What made me decide against them.
Edited by Born2bwire - 6/26/10 at 9:30am