[EdHolland]

You could also use a drill bit that is larger than the rivet. Proceed slowly until the head of the rivet is removed, and then the transformer should lift off. The remainder of the rivet can then be pulled out of the speaker frame

Of course you, must plan to replace the fastener when reattaching the transformer.. Check to see that you can get a screw and or nut into the space beneath the speaker frame without damaging the cone. 

Best of luck,

Cheers,

Ed