One might also consider that if the volume levels on a receiver are maxed out, or nearly so, the input of a normal amplitude signal may be enough to cause clipping in some section of the receiver. The receiver used may have some protection or limiting to reduce clipping on the amplifier section’s output to the speakers, but may not have such protection on a pre-out sent to a subwoofer’s amplifier. The subwoofer may not be clipping internally, but potentially is receiving a momentarily distorted signal when strong low frequencies are overdriving some portion of the receiver’s low frequency crossover or subwoofer output circuitry.

Certainly a decent calibration is a good next step in a series of checks before exchanging your subwoofer.