Speakers sound saturated, especially in the bass range.

Greetings: I have a 7-year old Harman/Kardon HKTS-16-WQ 5.1 Channel Home Theater System that I occasionally use (5-6 time a year) for my outdoor theater. When not in use, they are stored indoors without any exposure to the outdoor environment. All of a sudden during a movie showing, the sound from all 5 speakers (4 satellites: SAT-TS11 WQ, and the center channel: CENT-TS11 WQ) became saturated and the speakers were rattling especially in the bass range. I used another amp and various songs to confirm the bad quality of the sound coming from the speakers. Any help you may think what caused the speakers to go bust, are they repairable, and if so, anyone has any idea how to disassemble them since there are no visible screws on the outside. Thank you for any help.