Oy! That's DuoLock and you'll likely need a prying tool to dislodge the locking beads. I use that when I want to lock down something I don’t want to move in transit but that I might occasionally need to pry up to repair or swap out. It does a great job but it’s tough to free in a tight spot.
I didn't want it to move from vibration, and there was no clearance to put screws in there. Mission accomplished. It won't move.
I've pretty much concluded that the tweeter is dead. I pulled both drivers and cut them out of the circuits. The M130 properly measures around 6 Ohm (normal for 8 Ohm), but the tweeter has no reading. I then plugged the crossover (sans drivers) into the AVR and ran a test tone to it. I can see voltage on all inputs and outputs, so I'm pretty sure the crossover is good. I'll measure a functional speaker just to compare the voltage levels on the output.
It may be my fault. I accidentally left my soldering iron on, and it burnt the tip, so to finish up I grabbed my automotive soldering iron. I may have gotten lucky with the other speakers, but it's entirely possible I got the connections too hot and burnt the voice coil. I solved my soldering iron problem by biting the bullet on a Metcal (the same thing we used at Compaq). Now I just need a replacement tweeter. Never a dull moment.