Here's the bass response you're getting out of the MBQ according to WinISD: The -3dB point is around 55hz, which isn't awful, but the falloff is much too steep.
I would recommend adding a subwoofer in the trunk. Even a 6.5" or 8" sub and a 100watt amplifier would drastically improve the bass. You can try changing the baffle but you're probably not going to get the results you're looking for.