Hi Jakes
I have a E36 318is as well and also struggled to get the bass in the car, but a friend of mine has found a solution, its the box that makes the whole difference and is called a slot port box.
I have one in my E36 and TLvivo has one in his E46 and the sound is ten times better than a normal box. Both of us are still using the old Subs we had in previous boxes,
my setup is as follows:
JVC DVD Head unit
Standard speakers in the front - connected to the deck
6 x 9's installed in the back - connected to the deck
JBL 12" SVC - Connected to a Starsound Digital 1500w Monoblock
Total spent, R3500 or thereabout and i have decent bass in the car. Its still not as loud as it would be in a hatch, but compared to all previous setups that i had and that i have seen, there is more than enough bass.
heres a pic of the box, i can hook you up if you want one, (please mind the car, its was dirty that day

)