Epicenter proper connection=lower sub vol.?

[Felix llamas]

I have an epicenter by audio control, and when I had it hooked to the sub output from the aftermarket head unit, the subwoofer sounded louder, now that I have it hooked up to full range output from my 4 channel amp, it sounds less loud, how can I get to sound louder again or the same loudness it was before, I turned up the gain on my mono sub amp, but even then its not as loud. And it gives off this low distortion sound when the epicenter isn't even turned up