Just a friendly reminder to those who do not know. Most class D amplifiers have a low pass filter that gets rid of distortion associated with the class D design. Most are designed to have a flat frequency response with a 4 ohm load on the channels. When you drop the load to a 2 ohm or lower, you will normally have a 1-3db roll off somewhere between 10-20khz.
"The final, and no less important, section is the output filter. Most Class D designs utilize a Butterworth filter scheme for simplicity and low cost. The output filter is essential for low pass filtering, or integrating, the carrier's varying pulse duty cycle into the original audio content while attenuating (absorbing) the switching carrier frequency. Selection of filter component values is very important and essential to maximizing efficiency."