[Ron Ramirez]

That's odd to see that rust on the cone; does the chassis also have rust? What about the other side of the speaker frame?

I suggest Evap-O-Rust Gel, applied carefully. If you coat the front of the frame and leave the speaker with the cone facing up, I think it should do its job without affecting the cone. Then if the back side of the frame is also rusty, then carefully clean off the front following the Evap-O-Rust directions, turn the speaker so the cone is facing down, and repeat on the back side of the frame.