i used to be into all this bassey crap. i sikaflexed the farkkkkkkk out of my ke70 boot, running 2x12s. epsilon alpha, tough subs!
it still rattled, but the sika flex on anything that can move is great, stops it alot.
put little foam pads behind ur number plate and add extra bolts.
when sickaflexing, where latex doctors gloves, and carry a turps rag to clean it up as you go. if you leave the little stringy bits to dry you'll have a prick of a time trying to clean it all up.
once i got the 1g ke70 i never had a stereo in that car, nothing like listening to a 2 litre six pot turbo do 3000rpm down the highway