No the sub starts rattling at very reasonable listing levels so def not overdriving it. At this point I'm also pretty sure that it has nothing to do with the backplate but the chassis/body of the sub. That rattling sounds wooden. I talked to the German distributor and he was pretty sure what it was. If you reach into the sub and then down (from the back) there's a small black bracket/bar. Something like this (just the wood):
http://www.stair-rodsdirect.co.uk/assets/images/products/full/bracket%20brass%20front3.jpg
That's the part that's supposedly causing the noise.
He recommend to angle the sub upwards and pour a special kind of resin down there and leave it to dry for 24h. Gave it a try, will report back in a day.