If its the typical PE/Apex/Stryke style plate amp, it uses an EC SK 2nd order HP filter. Essentially its a high-Q 2nd order highpass filter. When Q>1.0 it forms a filter peak which is what forms the "bass boost", or EQ. Modifying it is a matter of replacing two resistors on the preamp board, and using simple textbook formula's to choose any Q and Fc you want.

It is possible to design a subwoofer which EQ is an integral part of the design to form flat response, but EQ used disappropriately always makes a bad result.