with class AB or B amplifiers, there have been commercially made amplifiers using both voltage feedback and current feedback in order to have some control over the output impedance. the Yamaha M-4 (or was it B-4?), had a variable output impedance control, and i think the range of the control was from -2 ohms to +8 ohms. the trick is to get the class D to behave in a similar manner. there's one catch, very few class D amps have adequate feedback for such "feedback magic". there's really not much open loop gain to work with. an analog amp can have a lot of open loop gain, usually somewhere between 60 and 90db, and when the loop is closed, and the gain is fixed at a voltage gain of 30 (for a 100 watt amplifier) and the "excess gain" (called gain margin) is what has the effect of eliminating distortion as well as lowering the output impedance. with a class D amp there is no gain margin because the gain depends on the amplitude of the triangle wave going into the comparator, and the output transistors' power supply voltage. you can get some gain margin if you use an op amp to feed the comparator with audio, and tap the feedback for the op amp from the speaker side of the filter.