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

I am a big fan of efficient speakers.  I don't understand why manufactures make 4 ohm speakers for example.  I believe you need more power to drive them.  If that is the case, I would imagine this in turn creates more distortion.  Not sure if I understand this correctly.  Just an observation I have had.  I would think higher efficient speakers can create higher amounts of head room as well.

4Ω or 8Ω speakers are a myth.

Just look at the impedance curves on Stereophile. Some go from 2 to 20 and beyond.

SPL efficiency @ 1w or 2.83v is also highly suspect as few speakers are 8Ω @ 1kHz.

I think this heat issue may effect high frequencies most.

And it may be that the AG sold, “Deulund-Mundorf Ultra Speaker Purifiers” could be the solution to the heat issue. After 100++ hours of breakin they have had a profound effect on the soundstage and imaging. I now hear layers in a wide and deep soundstage and imaging has come into focus like never before.

These can be self made but the seller adjusts the values of the cap and resistor to match your speaker’s needs. 
This idea is not new. They are also known as Zobel Networks.

They only effect frequencies 12K and above. So above the crossover point of most.

Radically improved my listening experience.

Welcome to the real world of ’power compression’. JBL ^{[ 1 ]} has performed tests showing that power compression can reduce output by anything from 3dB to 7dB from the expected SPL at elevated temperatures. Seven Decibels!

Two article just came to mind!

Mike

https://sound-au.com/articles/pwr-vs-eff.htm#s

Speaker testing often shows thermal compression at 86, 96 and 102 dB.
So if one is listening at 80-85 dB, and there is little or no thermal compression at 102dB, then that is ~20dB of dynamic range.

Hence it is something that, IMO, is worth considering as much as FR.
Along with the HD and IMD, it shows a lot to be concerned about.