The boot in an e36 has much sound deadening, well, I think its the case for most BMWs. To get serious bass you have to get a serious sub and amp. But a well set up system, by someone with good knowledge and experience, with the right components, you will have a kickass system.
I think my sound for my e36 came to about R20K and I wasnt satisfied. It sounded fantastic, but Im too much of a stickler really.
Upgrade the head unit, all the speakers around the car with some great splits and power them with a good amp, that will be the first step to great sound.