It is very much correct. A passive sub only needs a low pass filter.to set the point to what frequencies it receives typically somewhere between 40 and 150 Hz. The high pass filter is associated with the mains. And it can be adjusted within a range to fully drive the mains to their sub region or something short of that. Thing is some .1 Receivers, like the Marantz NR-1200, have no high pass filter function so the mains are going to run full out.This is incorrect. They require both: highpass to protect from over-excursion below tuning and low pass where you want to transition to your mains.