Shielding unshield speakers: Possible?

Take the woofers out and wrap copper sheetmetal around the magnet.

You can cover the speakers in mu-metal, a soft metal that shields against magnetic fields. The more completely the speaker is wrapped, the better, since magnetic fields tend to leak around edges. The thickness of your sheet depends on how strong the field is. You can buy mu-metal from vendors who have web sites, just do a web search for mu-metal.

Check Parts Express for"buckng magnets". They get epoxyed against the magetic force. This will make your speakers sheilded. I know Polk sells them also for their older speakers, Monitors and SDAs.