Bass is lacking

Hey guys ... I recently installed 2 - 12" MTX 6000 subs each on their own MTX 421D amp.  First off ... I am not very impressed with the sound and I'm trying to figure out what the problem is so I have a couple questions.  First off the subs are facing toward the front of the car (in a 2004 civic) and are about an inch from the back seats.  It kinda seems like I get some distortion because of this.  Anyway ... overall it just seems like the bass is lacking.  I like to listen to my music loud sometimes, but I don't just like loud bass.  I like full sounding bass.  When I am outside the car the bass sounds great, but inside the car it is .... well it's hard to describe but hopefully somebody knows what I'm talking about.

So ... recommendations would be great ... like different subs, amps, whole new setup (i am currently building a new enclosure out of fiberglass so the subs will face the back of the trunk and I am going to dynamat the whole trunk).  Thanks in advance for the help!

There are two kinds of phase in audio: electrical phase and acoustical phase.  You want both of your woofers wired in the same electrical phase.  This means (very basically) that both + terminals are wired to the + terminal of the amp.  If they are in opposite electrical phase, the sound from one will cancel out part of the sound from the other since they will be moving in opposite directions.  You also want to wire the subs in electrical phase with the rest of your speakers.

Acoustic phase has to do with the timing of sound waves moving around in your car.  Sound is an air pressure wave.  Again very basically, think of sound waves like ocean waves.  You want the sound waves from each speaker to "peak" at the same time at your listening position.  If the wave from your sub peaks while the wave from your main speakers is minimum, they can cancel parts of each other and reduce what you hear in your listening position.  This also often happens if a sub cone faces one direction and the port in the enclosure faces another direction.

The phase switch on your amplifier changes the electrical phase of the signal going to the speakers without having to change the wiring around.  This is useful to correct acoustic phase cancellation as it will "reverse" the wave polarity (positive and negative peak of the wave.) 

In your case, put on some music you know well and listen to it from your driver's seat at your normal listening volume.  Then switch the phase on the sub amp and go back to the driver's seat and listen again.  If it sounds different, you have either created or corrected acoustic phase cancellation.  Leave it in the position that sounds the best.  If you cannot hear any difference, then there is insufficient space inside the car for the sound waves from the sub to fully peak before they reach your ears. Low frequency sound waves are very long (50Hz is about 23 feet) so they require physical space to develop.  This is also why sometimes bass sounds "better" outside the car than inside it: the low frequency waves have the space they need to fully develop.  Try moving the subs so they fire towards the rear of the car and see if this makes a difference (this increases the effective distance the sound must travel since the wave must now bounce off the rear inside of the trunk before it gets to you.)