I had Bill at Great Plains Audio rebuild my Altec Lansing VOT 416-8 woofers two years ago. He reconed them with OEM parts and remagnetized the 2.4 lb alnico magnets. They were rated at 12,000 Gauss when new. If you drive the speakers hard the magnets will demagnetize.. These speakers were from the 50's and I still love their sound and prefer my Altecs to my Theil CS3.5's
When Alnico drivers have partially demagnetized you will notice a difference in efficiency and a noticeable loss in the higher frequencies. An alnico driver in need of recharging sounds kind of dull, and doesn't have as much transient jump. Basically, it sounds dull.
Speaker magnets can be regaussed by Bill at Great Plains Audio in Oklahoma City.