imaging above 200hz would be good with some eq-ing with the midbass that far away, it'll sound a little wierd. try to keep everything above 60-70hz in front of you to setup a decent soundstage. I'd rather see you do just 6 1/2s in the doors, and tweets in the dash(no rear 8s) and subs. then put your midbass back there.

I know you're just itchin to fill those holes, but think a little abstract, they make great places for fuse blocks, RDAT cover, strobe cover, grill with alarm sirens behind it, even just a set of 5 1/4 components with very little power running to them as a good rear-fill setup.

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My $.02
95 GS