Started experiencing this problem a LONG time ago, then blew my last Sundown, so when I got a new Sundown, I re-located my ground (which I thought was the cause of the problem).
Now, I replaced the Sundown SA-8 (w the same Kicker 500 powering it). At low volumes, the sub often won't play at all (the amp appears to be running as normal). This often happens right when starting my truck up. If I turn the volume up loud enough, it's like the sub will 'catch' and then I can even turn it down and it'll hit normally. But you can even hear a slight variation in how hard it's hitting sometimes; and if i take a sharp turn or hit a bump in the road, it'll cut out again. I really don't think it's the ground (it's definitely bolted down) but is there anyway to know for sure really? To me, the fact that bumps and turns effect whether or not the sub is hitting at all sounds like a ground issue; but i really don't think the ground is the problem.
Other things I considered include: voice coil (but if I press in on the sub it doesn't start hitting, so I figured that eliminated that idea), and also I have a STOCK battery from 2004 (but I figured if it wasn't putting out enough power then my amp wouldn't operate as normal?). I thought it was the battery on my last Sundown but we put a Multimeter to it and it was putting out adequate power. HELP!
Another note: there will also be times where it'll about "half-way hit", and you can tell it's trying to knock like usual but just doesn't have the power? Sounds like a blown sub almost but like I said, pushing on the cone doesn't effectively cause the sub to start functioning again.