First off, bottoming out a subwoofer is bad right? Like it will dent the voice coil and rub right?

Anyways, I have 4 MA Audio subwoofers (MA120XE, 12", 350 watts RMS) installed in a single box about 6.5-7 cu feet (before displacement). The subs hit hard, and I'm only pushing 1000 watts RMS to them just to be on the safe side. Turning up the gain some though obviously makes them hit harder, but I don't know if I'm bottoming them out or not. How can I tell?
Epi Gothic Thunderbird
Squier P-Bass w/new pups
Ibanez RG7321
Washburn Acoustic Guitar

POD X3 Live $300/500.