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To understand why permendur, as reported elsewhere and  to my own little  experience, sounds different to soft iron as conductor of magnetism in loudspeaker motors one must look at permendurs  properties.

Two differences might contribute: Magnetic and mechanical.

The mechanical first:

I tried to with a piece of wood to tap the central pole piece of two otherwise identical loudspeaker magnets, one with only soft iron in the magnetic circuit and one with permendur pole pieces.

The permendur one had a much louder and longer lasting ringing sound, that correlated well with the impression of the two units when playing music.

The magnetic:

Take a look at the BH curves of permendur.

One will notice that it saturates easy and hereafter reaches the very linear saturated permeability area.

As long as the permendur is kept saturated it is very good and linear (distortion free) and the pole piece of the Featrex is undoubtably saturated in the magnetic gap.

Following the field lines in the magnetic circuit away from the magnetic  gap, the level of magnetism will decrease and might eventually reach the highly nonlinear regions of permendur, and unless the whole magnetic circuit is saturated  a can problem occur:

If the level of flux in the permendur is on the sharp transition between saturated and not saturated, any modulation of the field originating from the signal in the voice coil, will produce harmonics of the disturbing modulation. This in turn will be reflected back as a varying driving force into the voice coil and further into the radiated sound as distortion.

Soft iron does not have this sudden knee in its BH curves and will therefore be immune to the magnetic distortion mechanism described above.

If  both, or any of these factors are at play with the Feastrex is speculative, but distortions can produce the impression of richer harmonics, more vibrant etc.

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