Subwoofers don't clip. They just reproduce the signal they are fed by the amplifiers. It's the amplifier that clips.
An amplifier reaches clipping point when it starts to emit square block waves instead or triangular sine type waves. These waves when amplified cause erattic coil and cone movment in your subwoofer which can cause voice coil overheating or mechanical damage. Most amplifiers use type P and type N Mosfet transistors. A Type N for the +ve speaker lead and a type P for the -ve speaker lead. The type P is a little slower that the type N transistor and iz only used for retracting the cone backwards. Normally this transistor induces the clipping.
The only way to prevent an amplifier from clipping is to either turn the levels down or increase load impedance.
Bear in mind that head units, crossovers and equalizers clip in the same way.