Audio Systems: 4ch car amp problem, infinity kappa 6x9, rca cables

Question
Hi, I'm new here and I'm looking for an answer that I couldn't clearly find on the internet or pre posted questions.  The trouble I'm having is I recently boughtt a JL HD 600/4 amp for my interior speakers and to me it seems that the front/door speakers aren't playing as loudly as I think they would with this kind of power.  my setup (all aftermarket if that helps_) is a Pioneer 690 head unit with Infinity kappa perfect 5.1 doors and Infinity kappa 6x9 in the rear.  Best buy installed everything, head unit, doors, 6x9, and all of the amplifier wiring.  however, the amp they installed was a different one, since then i personally put in this new jl amp.  I'm wondering if I mismatched the + and - wires from the speakers to the amp, the only knowledge I have of this problem is that the speaker cone would fire backwards/inwards and not forward/outward.  I have no 100% knowledge of what mismatched wiring sounds like.  
side note; my concern is only on the doors, the 6x9's sound perfectly fine to me.  the doors don't seem to have the strong punch or clarity that i think they should have.  they sound a little muddy or powered very poorly.  second note the amp gain is turned to half way on the doors.
extra note, the amp wiring is 4gauge and the RCA cables, for that year, were supposed to be top of the line
cheap wiring was not used

thank you for your time

Answer
This is an easy one!(?)
Even though they are the same brand, the main culprit is the relative efficiency ratings of the front and rear speakers.
The Kappa 5.1's are MUCH less efficient (89 Decibels at 1 watt) than the 6x9's (95 Decibels at 1 watt). This is a -6 DB difference for the doors and it is significant and for all practical purposes it is exactly why you are experiencing this problem.
You are basically hearing the door speakers at 1/4 of the 6x9 level at any given volume (note that if the door speakers are closer to your ears than the 6x9's are, you may hear them a tad louder than 1/4).
Without getting too deep into this, consider that every time you double the applied power, you gain 3 Db (as long as the speaker can take the additional power). This means that you will need exactly 4 times the power going to the 89 Db@1 watt front speakers to get them as loud as the 6x9's in the rear. Unless you 'undertune' the amp level(s) for the 6x9's you will blow up the door speakers trying to resolve this by applying more power to the doors!!
In the short term, I advise you to tune the system from its' weakest link for the best Sound Quality (probably the door speakers). Bring the 6x9's level down to get a good front stage and some rear-fill when you turn your head (not a LOT of rear-fill as it is now).
Other options might include re-aiming the door speakers to better suit the sound wave propagation to your ears or even choosing more efficient door speakers if that is an option.

The kappa 5.1 power range is only listed as 25-100 watts RMS so beware as you set up the levels/crossovers. This is especially important right now since they are so relatively 'quiet' already and sound as if they are lacking. k?

I hope this helps. It is a lot to grasp at 1st and an eager novice can burn a vehicle to the ground if not careful AND safe.
Let me know if you need any further assistance.
Have fun ............TAKE YOUR TIME.......and stay safe.


PS
To check the speaker phase, use a 9 volt battery on the speaker leads at the amplifier end of the wires. If the tested speaker pops outward from the non-power position, the lead hooked to the positive of the battery IS the positive to that speaker. If the speaker moves inward, the pos lead of the battery is hooked to the negative of the speaker. Be sure all four speakers are wired in phase (moving outward) Do NOT hold the battery lead on the speaker wire any longer than needed to see the movement!!
Correcting an out-of-phase issue would not fix all of the volume level mismatch problem but it never hurts!
You will notice that there is more bass on a pair of fullrange speakers when the balance is centered when the phase is correct (especially noticeable in a simple, two-speaker system). If the bass increases as you balance to the left or right channel, one speaker is out of phase and reversing either of their leads will cure the problem.

JM