The problem is that the amp does not have enough power to run the L7‘s.
He says he is running two 10“ 2ohm dvc subs, so he can only wire to 0.5ohm, 2ohm or 8 ohm bridged. There is no way that these subs are wired to that amp at 2ohms in stereo, you can only wire them in stereo at 1ohm and 4ohm (series and parallel). The most each sub is getting is 179 watts, which is 400 watts below RMS. I think you will find that your amp is just not powerful enough.
As for the pre-out on his h/u, you ever look at the input sensitivity on an amp? On the amp that he has the input sensitivity is 0.2V to 5V, so no matter how crap his h/u is it’s still going to drive the amp.
Get a better amp, mono block amp, wire the subs to 2 ohms.