They simply dry out. Dry capacitor has increased ESR (equivalent series resistance) resulting in poor control of the bass.
When ESR gets high capacitor gets hot from the current flowing thru it. Increased temperature increases ESR even more and eventually it goes into thermal runaway (avalanche effect). Before it happens though, capacitor will be at least very warm. If it is cold and sound is OK then leave it. Life of capacitor is in order of 50 years in normal ambient temperature, but each 10degC temp. increase cuts life of capacitor by 2.
When ESR gets high capacitor gets hot from the current flowing thru it. Increased temperature increases ESR even more and eventually it goes into thermal runaway (avalanche effect). Before it happens though, capacitor will be at least very warm. If it is cold and sound is OK then leave it. Life of capacitor is in order of 50 years in normal ambient temperature, but each 10degC temp. increase cuts life of capacitor by 2.