With time (and loud music) your loudspeakers might get broken and sound like if they would be not resisting the (even low) volume. The foam-like flexible membrane between cone and box might have aged.
Check the bass woofer for broken/loose part between near the foam/ soft part: it might be broken:
Add some primer to avoid the foam melting. Then apply as thin as possible the cyanacrylat based glue: the movement of the foam allows the music waves to be created. There must be some movement remaining…
You might that way extend the life of your loudspeaker.