i believe tubes near the end of their life may be suspect to sibiliance, too. try tube rolling.
i hear sibilance on a pair of magnepans mg-iiia with my dad's mid fi gear, so i put some resistors in the tweeter (which magnepan has a provision for in the back terminals). however, this was not a problem with my own gear. though, he does not use tubes, so i attribute this to his all digital mid fi gear and not to the speakers. have you resolved this yet?
i hear sibilance on a pair of magnepans mg-iiia with my dad's mid fi gear, so i put some resistors in the tweeter (which magnepan has a provision for in the back terminals). however, this was not a problem with my own gear. though, he does not use tubes, so i attribute this to his all digital mid fi gear and not to the speakers. have you resolved this yet?