The amplifier occupies the lower part of the BL8000 casing whilst the drivers themselves are mounted higher up, so it sounds more like something is loose than a problem with the drivers themselves.
Before starting to remove the frets to investigate further it is probably worth checking the simple stuff first if you have not already done so:-
- Is the vibration the same regardless of whether the connection plate is open or latched closed?
- Is there any movement between the speaker base and the floor (the 4 rubber feet on the underside of the base can be screwed in/out to level the base)
- Has the nut that secures the main body of the speaker to the base come loose? (carefully lay the speaker down on something soft to access the nut underneath and don't over-tighten this as there is a risk of deforming the conical part of the speaker if you do so, but make sure it has not come loose)
Apologies if you've already checked the more obvious stuff but best to start simple and work on from there.
Report back once you've checked the above and we'll go from there - do you know how to remove the frets? I'll talk you through it if you haven't done it before as it is easy to break the clips if you are not careful.
Kind Regards,
Martin