Quick read of manual leads to:

The HU is 2V outputs. As such, with 1.5V amp setting will clip.

Sub crossover seems to be set in HU - no need for amp crossover. Don't try to match or double up as it Is unnecessary.

Take out the 10dB EQ on sub amp or were you aiming to have that frequency twice as loud?

Sub sonic is used to protect speakers. If ported box place just below port frequency or if sealed enclosure start at 30hz and adjust down to where nothing is gained sonically by going lower.

It's very rare for a average car sub to produce good bass below 30Hz. So why amplify frequencies that sound bad or won't be reproduced? In large concerts I often achieve better sounding bass by winding up the sub sonic filter allowing the amp/speaker to produce bass with better dynamics.

To get decent bass to 20Hz and below you need good driver extension you need decent sealed enclosures, better sub drivers and bigger amps. I have a HT Velodyne which easily does 20Hz but it is 1200rms/2000 peak into a 10" sealed enclosure with over 2 inch excursion.

__________________
[COLOR=Black]Liberty GT Spec B tuned 240 HP & 372 Nm at the hubs.[/COLOR] "Instead of banning high performance cars - ban low performance drivers"