Without knowing your system this is a near impossible diagnosis. If you have a tube based system, one of the first signs of a retube are a loss of dynamics. So if you have any tubed electronics I would try retubing unit(s). Also, I would check ports to see if there are any obstructions to ports if your speakers are ported. Also check phase on your speaker terminals. If any of your components invert phase that could change the dynamics of the speakers. Alas, you may have a voltage problem out of the socket so these are a few things off the top of my head that may cause problems.
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.