Maybe the drivers have loosened in the cabinet ? You could test the tightness of the bolts/screws holding the drivers. This is just a guess as I am unfamiliar with your speakers, but I have known this to happen on other models.
B&W 802 S3 losing snare drum snap
without changing anything to my system, my speakers have lost their mid range snap. very noticable on snare drum fequency. everything else is crisp. fearing loss of hearing, i invited a couple a friends over that know my systems sound, and they noticed it too. has anyone else cured this problem. i isolated all componets and cables. no help.