If you can leave subwoofer at +6 without clipping, leave it.
You can play with the HU eq a little to get the sound you want, just don't go maxing anything out.
On the 175.4, there should be a switch for the crossover, hp-through-lp or something. Set it to hpf and the lpf knob does nothing. So just set the hpf knob to 80-100.