The BOSE amp shallows out the bass to maintain accurate response. As you increase the volume, you exceed the amplifiers ability to accurately reproduce the bass. So rather than clipping and crackling like most aftermarket components, the BOSE system plateaus the output to the offending speaker.

The only way to eliminate this is to purchase a 5channel aftermarket amplifier(or directly wire the speakers to your headunit and purchase a class D subwoofer amp) and wire it into the bose surround system. Because the bose system uses 2ohm speakers, the results will probably be less than satisfactory and will require new speakers as well.