Ok, I started from scratch. with a fresh battery in the DD1. My head unit's volume goes to 40. I tested at the amp, and playing the 40hz tone, the head unit distorts at 28, so I wrote down 27, then playing the 1000hz tone I can go all the way to 40. I averaged the two and used 33 as my volume to set my amp gains. The system sounds good, but I'm wondering why I get head unit distortion at 40hz and not at 1000hz? Is this something to be concerned about?