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I get the whole Basson thing now. Impedance isn't static. It changes as you play. And if you're not giving the cab enough juice for the speakers to show any impedance at all, it'll go into infinite ohms and blow the OT or overheat. Totally makes sense now.
That can only be done with sophisticated measuring gear.
I believe you mean to say that impedance isn't constant. An 8 ohm cab, for instance, may measure anywhere between 5 and 100 ohms, depending on frequency.
No. The impedance of the speaker is there whether you're playing or not. OTOH, the DCR component of the impedance of a driver does increase significantly when the voice coil heats up. The threshold of this increase usually lies around 100 watts per driver; above this threshold the increased DCR reduces the sensitivity of the driver. This is called Thermal Power Compression, and can be so severe that additional power input garners no additional output, only more heat, and therefore more DCR. The usual result of thermal power compression is blown voice coils, as long term heat build up eventually causes the adhesives holding the coil together to fail. But in the case of a tube amp that long term heat build up and the continually rising DCR that accompanies it could also explain output transformer failure.
