Generally the bigger/higher gauss the magnet has, the better. Yes over time the permanent magnets do get weaker, but they can be reenergized. I still have some Altec and JBL from the 60-70's, great speakers, Alnico magnets. The newer Neodymium magnets are great and very hard to demagnetize, expensive, but a little goes a long way. Look at the gauss rating if specified, helps control the cone movement.