I have reduced the output of the tweeter in my M3 a full 3db. This has kept my M3's from sounding like the worlds largest tweeter (my wife's comment.) You can do this by placing a 2.4 Ohm resistor in series to the tweeter and a 20 Ohm resistor in parallel to the tweeter. This will greatly reduce the brightness, harshness and fatiguing problems with this tweeter. I turn up the treble knob on the preamp to add some sparkle and life above 10KHz. I have found with the resistors and a slight increase of the output above 10k turns the little M3's into a very nice warm sounding reproducer.
