I am beginning to think that perhaps not only does amount of space in the trunk available for box matters, but also the amount of free air space in trunk as well. (ie large box covering whole trunk is no good)
I ,too, am questing for 150dB. I drive a cutlass ciera with about 6ft total airspace available for a box. I am going to look at 2 juggernauts 12" each powered by an usamps merlin d2.
Looking a lots of dampening material to keep sounds inside the car, perhaps replacing worn/brittle weathertripping. Deaden inside doors. Most people will reccomend test and tuning. I am going to see if any local shops rent termlabs.