um what? since when does coil impedance change loudness all other factors aside? low impedence needs more electrical to keep up up with it because it's putting out more power not less. The biggest difference in impedence loads is the dampening factor of the amp, Not THD. Now where it gets hard is if the higher impedence drivers have smaller tighter wound strands on the coil, then it's efficiency may go up. Impedance isn't what determines that though.