Speaker compression is not a myth. The voice coil gets hot. Very hot. The resistance is proportional to the temperature. As the temperature increases the resistance increases. The tempco of copper is about 0.4%/deg. C. If the voice coil temperature increases, say, 50 deg. C then the resistance of the voice coil will increase by 20%. If you doubt this think about how hot a 20W resistor gets. Or a 50W. 99% of the power delivered to a speaker is dissipated as heat. The 1% left is acoustic power. If you put 50W into a speaker 49.5W are converted to heat in the voice coil.
For simplicity let's assume that the speaker is driven by a power amplifier with zero output resistance. The power into the speaker is Vrms^2 / R. If the resistance increases by 20% then the power decreases by about 17% (roughly 1.6dB). That's very audible. Voice coils can actually get much hotter than this, even above the boiling point of water. Some speakers are rated to 150 C or more. That's why they use Kapton formers as plastic formers would melt. Most loudspeaker failures are usually due to voice coil overheating.
This is modeled under the Dynamics page of the Amp block. You can set the amount of compression (voice coil temperature increase) and the thermal time constant. The default values are typical of a 12" guitar speaker.
Guitar speakers also exhibit nonlinear impedance characteristics. Guitar speakers are designed so that the voice coil exits the magnetic gap under normal use. This causes compression when driven by a tube power amp because the magnitude of the impedance decreases and therefore the voltage from the power amp decreases causing a gentle compression. This is modeled by the Speaker Compliance parameter.
Finally guitar speakers exhibit nonlinear distortion behavior. There are two types of distortion. These are modeled by the Speaker Drive and Speaker Thump parameters.
