If the speakers are rated at 6 ohms, it's a little tricky figuring out the individual driver impedance. The tweeter could be either 6 or 8 ohms. Maybe Mirage could supply this information. I suggest trying an 10 ohm resistor in case the tweeter is 8 ohms. If you find out the tweeter is 6 ohms, you could switch to an 8 ohm resistor. It will work in either case, if this is in fact the problem. A 0.47uF cap should do do the job in either case.
The way this works is by putting a load in parallel with your speaker. The resistor limits the rising (w/frequency) impedance of your speaker. The cap determines the frequencies that are affected. So you want a cap that will cut in beyond 20K - the smaller the value of the cap, the higher the frequency. The resistor value should be greater than the impedance of the drivers. If this works you can tweak the values of the components. Generally speaking, 2 ohms greater than a drivers impedance is a good place to start. With the cap value - a lower value will keep it further from the audible band, but could be less effective. This worked for me with one of my transistor amps that has response beyond 100K. It was unlistenable with a certain transistor preamp until I tried a zobel. With a 6 ohm tweeter I'm using a 0.33uF cap and a 8 ohm resistor.
The way this works is by putting a load in parallel with your speaker. The resistor limits the rising (w/frequency) impedance of your speaker. The cap determines the frequencies that are affected. So you want a cap that will cut in beyond 20K - the smaller the value of the cap, the higher the frequency. The resistor value should be greater than the impedance of the drivers. If this works you can tweak the values of the components. Generally speaking, 2 ohms greater than a drivers impedance is a good place to start. With the cap value - a lower value will keep it further from the audible band, but could be less effective. This worked for me with one of my transistor amps that has response beyond 100K. It was unlistenable with a certain transistor preamp until I tried a zobel. With a 6 ohm tweeter I'm using a 0.33uF cap and a 8 ohm resistor.