The second biggest contributor to fatigue that's not power per say, is electrical resonances/ringing.
The most free fatigue speaker I found was the late Carver Amazing ribbon, due the time propagation from the kapton/alu tape.
This ribbon has a propagation time perfectly according to the human brain, hence the real 3D sound stage.
Dome/cone sound propagation time are not what the human brain expect, hence the cold, unnatural sound presentation.
The Julian Hirsch Stereo Review was incredible.
