Driving piezoelectric crystals, low to high or matched impedance?
It depends on how the amp is connected to the load...

If the cable is short enough relative to wavelength not to be a transmission line, and its inductance is small enough, then you will get maximum power into your load by using a low source impedance.

However if your cable is long and becomes a transmission line at your frequency, then you have to match impedances, or else power will be reflected at the load.

So, both answers are right, and the deciding factor is the cable length.