The left channel from the pre amp goes to the left sub and the left channel from HPF will go to the left channel of amp. The right channel from the pre amp goes to the right sub and the right channel from HPF will go to the right channel of amp. BTW, that wyred 4 sound sti500 does not have pre out and in. These are the instructions for the high pass filter!
https://hsuresearch.com/products/manual/HighPassFilterManual.pdf
https://hsuresearch.com/products/manual/HighPassFilterManual.pdf