Thermal Distortion your loudspeaker most likely suffers from it. But do you care?

''Alnico magnet’s were/are sometimes prone to demagnetize:''  Yes!

My understanding is: The JBL LE15A 15'' Alnico woofer has a top plate of more then 1/2 an inch thick, short coil, long magnetic gap, the flux density in the voice coil gap is 11000 gauss, hence it will not demagnetize itself when hit hard with high level input power. That's one of the reason why its considered to be the best vintage 15'' Alnico woofer JBL ever made. Also Alnico is the best for making permanent magnet speaker drivers, Alnico stability and resistance to back EMF is really good. JBL engineers discovered that more then 75 years ago. That is the reason why JBL chose Alnico 5dg magnet for the JBL1501Al-2 woofer, not Ferrite nor Neodymium. Enjoy! That's what it's all about! 😁 

IMO Ferrite is basically a lousy magnet material for speakers but it is cheap and readily available. 

Mike

''The new 1500Al used in the S9800 can take continued pulses of 5000 watts and loose no more than 1%. The test can only be done a few times before the coil is destroyed, but the magnetic assembly is totally stable.''-Greg Timbers JBL engineer

http://www.lansingheritage.org/html/jbl/specs/pro-comp/2215.htm

http://www.atma-sphere.com/en/resources-common-power-amplifier-myths.html

 Matching the type of speakers with the best amplifier makes a huge difference. 

I concur.

Mike

The interesting thing about Alnico magnets and compression is that some theorize that the particular sound of Alnico magnet speakers is that they are actually subject to more compression than other magnet types because flux density is lower under dynamic conditions when the voice coil is excited. 

If high efficiency speakers are so darned good, why did the industry move away from them? So electronics makers could sell more expensive Power? Methinks not.

@ieales 

The industry moved away from higher efficiency because its expensive, when solid state power became available. Sometime during the 1960s it became obvious that transistors were substantially less expensive than tube power. So with the less expensive power came speakers that were less efficient because (no surprise here) the speaker manufacturers could make more money. As tube popularity waned, output transformers and the tubes themselves became progressively more expensive; meanwhile solid state power got cheaper.

But prior to all that, the industry moved away from field coil to permanent magnets, not because permanent magnets were better, but because they were cheaper as well! In a similar way, CDs were cheaper than LPs on both the record side and playback side.

Whenever you see movements like this, in audio traditionally its always been about increasing profit. There are things that fly in the face of this a bit, for example some manufacturers have learned how to reduce thermal compression in less efficient drivers by proper venting of the pole piece, allowing for greater cooling (although things like that can be applied to higher efficiency too...); IOW they are trying to improve the product rather than make more money on it. But that sort of thing is rare.

The question was rhetorical 🙄