for a start, dont fit the speakers in the shelf.

The air movement from the subwoofer can affect the operation of the 6x9's in the shelf.

You'd be better with a good quality front set up, rather than a poor quality rear set up.

I have used velcro (a fair amount) in the past to stop the sub box moving around, and fitted the amp to the box, or you could fit a wooden floor and affix both the amp and the sub box to it.

hth.