Simple means "low quality".
A filter for a subwoofer is usually about 100Hz to 250Hz, not way down at 40Hz.
The inductor in an LC filter at a frequency as low as 40Hz will be huge, expensive and ruin the damping provided by the extremely low output impedance of the amplifier.
Use an active lowpass filter that is 3rd-order (18dB/octave) that drives the power amp that feeds the sub-woofer.
You also need similar highpass active filters to feed the higher frequency amplifiers.