Mickey, If the nibbler doesn't work for whatever reason, maybe just you or your installer follow the instructions on the last 2 pages (starting with step "21") I pictured above on cutting the center clearance whole. Your new speaker should have come with a template to locate it properly and match the center opening size needed in the panel to allow the new dual speaker magnets to recess and let the speaker sit flush on the surface. If not, you may need to create your own template.
For actually cutting the center hole, the original speaker instructions steps 24 and 25 say:
"24. Cut out the two center holes with a 3-1/2" diameter hole saw." (Double check the dimensions of your dual speakers to make sure the hole opening is enough to allow for full recessing into the panel.)
"25. Cut out the two triangles of metal between the two center holes."
Let us know how this turns out.
