Low pass is usually good around 80HZ (60HZ if you only want the real low sounds going to the subs)

The slope is probably how wide a range of bass is boosted, so the higher, the more bass response you get at the level you set it.

I had an alpine that had Low Pass Filter(LPF) and BassBoost adjustments. The LPF was set to 80HZ And the bass boost range was at 60HZ and expanded by 4 in either direction. Not sure if that is 4HZ, it was just the biggest range to the bass boost my head unit had.

I hope that helps a little bit.

Sorry I'm not to familiar with the High Pass Filters to run any info for that.