If you're lucky, it might be a loose screw. Just in case, check to make sure the screws are snug.
Imo the loose screw thing is a long shot - my guess is that the midrange driver has been damaged by overheating. You may not have sent too much power into it - it may have been defective all along, and finally failed.
Too much heat (i.e. too much wattage) will soften the glue that holds the voice coil wire together, and some of the coils will detach from the voice coil former and/or from the other coils. Typically these are the coils at the outermost (forward) edge of the voice coil, as there is usually less nearby thermally conductive mass (the motor assembly) to act as a heat sink.
Or if the glue job was uneven to being with, there may have been a weak area that failed when the glue got hot from the voice coil heating up.
IF you are comfortable with taking your trouble-shooting endeavor to the next level, remove the screws holding the midrange driver in place and see if you can remove the driver. If you can, then hopefully the wires going to the midrange driver can be easily disconnected. If they can, then make note of which wire goes to which terminal, and disconnect them. Then remove a good midrange driver from one of your other Jamo Concert 11 speakers and install it in that speaker. This should allow you to test whether the distortion goes away, and determine whether the midrange driver is bad, or whether it's something else.
Eyeballing a photo of the Jamo 11, it looks like the midrange driver has
a fixed "phase plug" rather than a dust cap, implying that there is a
slight gap around the phase plug. If you establish that the problem is definitely in the midrange driver, it is possible that a bit of dirt or
debris or perhaps a bit of ferrous metal got into that gap. If MIGHT be
possible to blow debris out of the gap with a blast of compressed air, like what they sell for cleaning electronics. I don't know what the risks are of air-blasting the gap around your midrange driver's voice coil, but if it's already "broken", well, you've probably got nothing to lose.
Best of luck to you.
Duke
