Let's not forget that when you replace speakers, you should do so taking into account the crossover and frequency ranges of the speakers. This is going to be vital to the performance of the sound system.
I'd be extremely interested if someone has a tone generator to find out exactly what the crossover frequencies are in the MDX. That will give a good indication of what speakers to use based on the speaker's specifications. Or, if the crossover frequency is adjustable. A good sub should be able to go right into the almost-inaudible range of 20 hz.
The best test of any subwoofer is playing the Sting CD, "Brand New Day". Track one on the CD, for about the first 30 seconds you should hear nothing but chest-rumbling low frequency harmonics. If you can't hear a thing, or hear very little, your sub is not doing the job. My Velodyne subwoofer at home will rattle the foundation with that test
