I've never seen a torque spec for a subwoofer. It probably depends more on the material you're attaching it to than anything else. Really all you're trying to do is hold it in place and keep things tight enough to maintain the air seal. 100 ft/lbs is hopefully a joke. If anyone actually measured it, it would probably be more like 100 INCH/lbs., which is about 8 ft/lbs.
Just put a foam seal (like weatherstrip material) around the edge of the opening and tighten the screws until it flattens out. That should be tight enough. Check it in a week to see if there's any looseness. If not, you're fine. If so, snug them down a little tighter or use bolts with locknuts. And if you listen to it REALLY loud, get used to tightening things back down all over your ride!