Speaker magnets

Field coils have been around a long time.  They add a lot of weight to motors and generators/alternators.  A 150lb starter of old is 40 lbs at most now. Fixed neo magnet starter motor, the solenoid is almost as big around as the motor.

They say FC speakers are some of the best ever made.  They have to be expensive to make too. Interesting critters!!

Regards

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. 

An FC mag puts heat around the VC something you don't want. It adds needless complexity and for some strange reasons sounds the best powered by a Tungar DC power supply. The older FC designs were overbuilt but I wonder if the heating-cooling cycles an FC produces would affect the longevity of the FC in modern FC transducers. That being said FC, Alnico, rare earth, Iron ferrite mags all can sound fine depends on the overall design of the transducer and imprementations when used in a finished loudspeaker.

There are magnets and then there are magnets.  The older speakers either used Ferrite or Alnico magnets.  The alnico magnets were used in some of the better speakers at the time, like JBL.

From what i understand the size is not necessarily the thing that make it better. For example High Q driver used in Open baffle designs can have small magnets but smaller gaps in the voice coil (closer tolerances from what i understand are part of what makes a higher efficiency driver hi efficiency).
 Magnets got really big in the desire for big bass from small drivers to get the control of the cone and maximize excursion (why small speakers usually need lots of power and damping). Large drivers may not need the excursion so the magnets may be smaller. so magnet size is not a factor in sound quality from my experience, its all in the overall design and intended use. 

Maybe a speaker designer will chime in and clear some of this up.