[SteveR]

Well, 6.5s aren't really bass speakers. You can get them down to about 150Hz with less than 3db loss. Check the specs on the speaker and see what it can do (low frequency, and SPL). If you have a crossover, try adjusting it so you are sending the lows to them. If at high volumes the speakers and/or rear of the Jeep is rattling, you're pushing some decent low frequency response. If not, the speakers aren't even being driven hard enough. Whatever you do, don't push the speakers past their specs or you'll tear the rubber mounting off. If you are getting some rattling, try a few things; try using plastic washers where the speakers mount to the Jeep, you'll get some vibration isolation that way. If you think you're getting signal loss and the bass is exiting through the roof/window, pad the back of the roof or whatever is behind the speaker with something that has a very non porous surface. Something that has a harder or reflective surface. The point being that if you use like insulation, it will absorb the low frequency waves and you'll get even less bass. A reflective surface will reverberate the lows back into the passenger compartment.