[bobgpsr]

If you only have one subwoofer, then normally you only use one channel of the Cleanbox. So you only need to modify one Cleanbox input. The input coupling cap is the only real freq rolloff issue area because of its original value into the moderate input impedance forming a voltage divider. The output coupling cap is not a limiting freq factor due to its much larger value.

If you really want a moderate high pass filter -- then yes leave the Cleanbox unmodified. But those of us who are wanting real output at 15 Hz (or even down to 10 Hz) will want to do the modification to the channel(s) of the Cleanbox being used.