Bucking magnets work fantastic; just make sure the magnet is not bigger than the speaker magnet itself. I have installed two car speakers (30w rms) in an older cab. The speakers are about 10cm from the monitor. The combination of the magnets and caps are amazing: no distortion whatsoever. Be sure to glue the bucking magnets in place, though.
Oh, and having extra metal between the speaker and monitor is not always helpful: strong magnets can magnetize (part of) the metal sheeting, worsening the magnetic influence on the crt..