The M-20 amplifier can drive the Three with about 200W. Plenty of power to blow the woofer across the room. The problem is that the M-20 amplifier has eq built in for the M-20 speaker. There is a high Q (boosted) high pass filter and a shelved high frequency eq. It would not be too difficult to remove them from the circuitry. Cut a few traces and add a few jumpers. You will also have to remove the portion of the passive crossover that is in the M-20 connected to the XLR speaker output. This is very easy to do. If you need schematics and instructions, contact me by the ACS website.
If the system doesn't have the correct amount of punch, then there is probably some phase cancellation between the Threes and the sub around the crossover. You will need to take some acoustic measurements in your room to fix this. If you don't have any acoustic measuring equipment, go to www.hometheatershack.com
, join, do a search for REW (Room EQ Wizard). Download and install it on your computer. With a mic, it allows you to do acoustic measurements with the soundcard in your computer. It is very easy to use and works very well.