As the drivers are outside the amplifier (unless included in some sort of feedback), they have no effect on the amplifier, Back emf from the bass driver won't get through the crossover to affect the tweeter, and anyway, will be hugely attenuated by the amplifier's low output impedance, so there's no effect there.
As nothing is perfectly linear, nor is the amplifier's output impedance actually zero, there is always some residual effect of non linearity, but with any even half decent amplifier being 99.99% linear or even 99.999% linear, any non linearity can be safely ignored.
S
You seem to have dodged the question. I think the system in question comprises of both the amplifier and the driver. Using your example from Wikipedia, the input A is going into the amplifier and the response X is comming out of the speaker. If either the amplifier or the speaker is non-linear, then the system must also surely be non-linear and the Superposition Principle would not necessarily apply?