In discussions of this sort it is essential that we delineate linear distortions from non-linear distortions. Frequency response aberations are linear.

The horn or waveguide dominates the frequency response of the system, although there are a couple of driver effects. One is the resonance of air arround the voice coil - this needs to be controlled - and at higher frequencies there are diaphragm resonances. But basically compression drivers are far flatter devices than piston sources. This is because of the high damping both acoustically and electro-mechanically.