As I recall
, mu metal is what is used in commercial
magnetic shielding. It is a metal sheet that has a very high magnetic attraction, so it absorbs magnetic fields (energy) and prevents them from affecting whatever is on the other side of the mu metal. Stainless steel
is a weak magnetic attraction at best, you want something that leaps to the magnet and soaks it all in.
Mu metal suppliers can be found on the web. Some quite pricey, others infest alibaba. I would expect that any "loud" or "powerful" speakers would be more problematic, because they'll be using bigger stronger magnets to handle high volumes. The old speakers may have been chosen for economy, making that less of a problem.
If you do decide to go speaker shopping
, take a hand compass along with you, and use that to check them out in the store.