basiclly if you put rear speakers in then youl get the high stuff blasting right in your ear as if you had your back tuned to the band so normlly you fadar them down or get dont have em (mine are turned right down)
with on the sub , the Z is small and if your nod fater a big car shaking ratle then do like i have .....
viewtopic.php?f=27&t=28201
see finished item although i may move this to the compartment side of the strut bar so i cna usea tennaeu cover .
an 8 or 10 is more than enough to provide good bass .
if your handy with making items then id make small sub box for the boot and try and bag a nice 4 channel but.. has your car got the orignal bose amp still inplace or was it non bose ??
this makes abig differnce as if its non bose id just use the HUamp to run the fronts and get small amp for the sub , that show run most of my set ups in the past (got the bose amp and an alpine for the sub now).
its all budget realted really
but a good absic setup is HU , 4 chan amp , pair comps for the front and sub size/power to suit tastes.
hope this helps , im sure some fo the others on here can help.