1- The L12 doesn’t contain a filter because it uses the economy Rythmik plate amp, not their full-feature model, which does.
2- However, picky listeners have found that many subs contain high-pass filters that aren’t the most transparent around, including Rythmik’s. Sub filters are not designed and built to audiophile standards, containing OpAmps and other nasty components.
3- There are two good ways to add a high-pass filter to your system:
a- Install a capacitor of the correct value on the input jacks of your power amp. That creates a 1st order (6dB/octave) roll-off at the frequency determined by the capacitors value. That value, in combination with your amp’s input impedance, results in the corner frequency of the filter. The formula for determining the appropriate capacitor value to achieve your desired x/o frequency can be found all over the net via a Google search.
b- Install an active electronic cross-over between your pre and power amps. A good one is the First Watt B-4, which Reno Hi-Fi sells for about a grand. It provides 1st/2nd/3rd/4th order filters (6/12/18/24dB an octave roll-off) in 20Hz increments up to about 1250Hz (I believe). Nelson Pass designed and builds it with all discrete components, no IC’s or OpAmps. The best deal around in an active x/o!