well, i know i wont be very much help, but i'm running the same shit as you. a bx1500d and 1 15" l7. i have my gain set a little below half in a 3cf ported box--32hz. if i turn mine up really loud, mine starts to bottom out. but, that's past my normal listening level. do as everyone has suggested over and over, try the elimination thing. nonone seems to know why it's louder at 450w.