It's true the front door speakers are low range speakers and the sub is for the real low ranges. My theory is that both the front door speakers and sub have too much strenght in the 100 Hz range, causing a double amplification of those freqencies, which in turn is experienced as the "boomy"" sound of the system in general. The fix we are going to apply (test) is to have the sub start decreasing from 50Hz and the front speakers be at full strength at 200Hz. The cross over point should then theoretically bring the total sound amplitude line in balance.