I didn't say they don't have any, just saying that it doesn't look like a problem to me, and other dynamic cans have more. Is what you're talking about back EMF, or simply that the amp has to do work? Watts is work even into a resistor. If what you say is true, impedance should vary at different frequencies --- when you get close to resonances inherent in the geometry ect. Even if I agree that there's impedance due to the work being accomplished, the impedance is still flat at 50 Ohms. Back EMF in normal dynamic driver headphones exists more strongly near the driver resonance and shows up on both impedance and phase graphs.
I don't hear anyone saying you need a transconductance amplifier to drive planar magnetics, so it seems to me it all really boils down to having a low output impedance and a big enough voltage swing. That's "same as it ever was" AFAIC.