i don't have a z4, but here's a suggestion that i did with my old mr2. on 2-seaters, space is at a premium, so you probably don't want to fill the trunk. what i did was put 2 10" subs in custom boxes right behind the two seats. the mr2 has a small shelf in the back that made it possible - silipin mo yung likod ng seats mo kung pwede.

then i only had to put the amp in the trunk and keep most of the space. only problem is sagad na yung 10" (the dealer even told me that 8" was the max that could fit but we made it work), and you can't move your seats back all the way if you're super-tall.

Quote Originally Posted by ILuvDetailing
You can go the all JL route but medyo pricey. hehehe.
or
Get Focal Seps, Precision Power 5-channel Amps and JL Subs.
lol...mas mahal pa ang focal/pp kesa sa jl eh