Probably the best solution would be to get a stealth shelf - it's an MDF shelf that replaces your parcel shelf, with holes pre-cut for you to fit your 6x9s into, this would mount them at the rear of the car and if your going to be using them mainly for bass then this would be pretty much ideal. They come tailor-made for each car and typically cost around £60.00.
The great thing with the stealth shelf then is that looking through the rear window, it looks like a blank parcel shelf with no speakers in it, it's only when you either open the boot or crank the system up that you'll know they're there.
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