Davey
This may require a thread of its own. The subject is very interesting but understand that int he case of a subwoofer using the speakers output, that is the signal is taken from the amp output and the subwoofers only reads the signal from the amp, the input impedance of the sub is very high, high enough not to load the amplifier, IOW the amplifer from its point of view, sees an ope circuit. it is not really driving it. while 4 Ohms is an issue for an amp, 10,000 is seen a s an open circuit... Not a problem for most amp: A high impedance load in parallel remains for all practical purpose a non-load. Same as when you put a VOM across the speaker terminal, the behavior of any sane amp shouldn't change at all.
meanwhile the amp is still feeding current and voltage to the speaker even if the response drops to nothing. The elements/drivers of the speakers do receive this power but can;t do much with it.. It heats up the voice coil or other elements and that has negative effects on the behavior of the particular driver or drivers.