You cannot make the magnet stronger than its specification. However if the charging pulse is too weak or too strong (overcharging) the magnet will not reach the specified magnetic flux.

Because of this one should measure the gap flux after charging. This is why the cone (or diaphragm) should be removed during charging. You cannot measure with the cone still mounted.

If the guy doing the charging is experienced, he will probably put just the right amount of charge into the magnet with the cone still mounted.

I have read somewhere that low frequencies will discharge alnico magents. So cone speakers might need recharging. Compression drivers will be less affected. I had two tweeters, four 1" drivers and a 2220 checked. Only the 2220 needed recharge.