Mr.Lahey

Being new to car audio, i don't understand what "gain overlap" really means, -10db, -5db, 0db ? What is the best way to tune my system with my DD-1? And what settings should i set all my dials in the amp? This is my first time using this and I don't want to misunderstand steps from the manual. 2sundowns U12 D4 wired at 1ohm Skar rp4500 amp LC2I ACR1 (bass knob) for LC2I