Impedance/bridging advice for JL e6450?

???Rear mids lowpassed at 100 Hz?  Result:  very little range and output in the rears.  I didn't do a search for that amp to look at it, but here's some general advice.  Load the amp at 4 ohms for all speakers, exception can be a SUBwoofer.  Don't expect mids in the rear shelf to do what a subwoofer would do (your post indicates that that's your idea).  If you want fronts and rears in your car, use a 4 channel amp so that each channel powers a speaker (or component set).  Use the high pass crossover for all four speakers;  a SUBwoofer would use a lowpass.  Don't get creative with wiring speakers together so that you can make use of an amplifier;  instead, buy enough amplifiers so that you don't have to get creative.  You can download the tech manual for the amp online.  Read it completely for answers to your basic wiring questions.

A 5-channel amp would provide for 4 speakers and a sub.  A 4 channel and a separate sub amp would do the same.  If no sub is to be used, adjust the high pass crossovers to get as much clean low midbass sound that you can get.

Some system design notes, and these are just known facts, regardless of how you are interpreting the specs of the equipment:

• The mid driver does not have a flat response down to 50 Hz.  It is rated at 48 Hz at -3 db, which is considered the lowest useable frequency.  At best, 50 Hz is 3 db down from the reference point, which is around 100 Hz and up.
• This amp provides a diagram for bridging speakers.  But when bridged, the amp will provide output into 2 ohms per channel.  Note that amps are tested and rated at the higher 4 ohm per channel load.  The specs deteriorate sharply in the higher (than sub) freq ranges when used at 2 ohms.  Sound quality will suffer.
• The continuous power handling of these speakers is 70 watts.  The output this amp provides at 45 watts into 4 ohms is a good match for them.  Driving the amp at 2 ohms will not only deteriorate the sound quality but be too much power for the speakers (even though it looks like it is very close).  The big difference here will be the sound quality at the 2 ohm output.
• If bridging were used with this amp, it should be for subwoofer use, not mids and components.  The amp was designed to be versatile enough that you could use a variety of combinations with it.  But still, keep the basics in the forefront of your design:  components and 6.5 mids should be powered at a 4 ohm amp load and both use the high pass filter.

With your choice of JL, the notion that you are looking to get power over quality is not considered to be the issue.  You, instead, should be concerned with how you can get the highest quality output from your equipment.

If you don't have ample damping (Dynamat, etc.) in your front doors, on both panel and door skins, you are losing a lot of potential bass response.

I am not changing anything I said in my initial response.  A 6.5 woofer by any manufacturer has its limitations as far as frequency range goes.  Consider 80 - 90 Hz to be the lowest freq (of equal amplitude to the higher freqs), and that freq response is enhanced with the reinforcement of a subwoofer. 

Use the high passes, load the amp at 4 ohms, damp the front doors and the rear shelf.  And add a subwoofer with enclosure to reinforce the 120 Hz and down response.  This amp will indeed provide for this entire system, as a sub can be bridged to channels 5 and 6.  When this system is set up correctly, you'll find that 45 watts X 4 plus the sub output will provide a very loud, very clean sound.

100Hz is, in general, too low for the LP on your mid-bass. assuming I understand your intention of reinforcing the mid-bass.  The mid-bass region is generally considered to be between 80Hz and 320Hz, and some people cross as high as 700Hz although that is well into the normal mid-range band.  I recommend you high-pass your front mains around 240Hz and low-pass your mid-bass at the same.  Or better yet, band-pass your mid-bass between 80Hz and 240Hz and set your subwoofer low pass at 80Hz.

Just my 2-cents.

As far as utilization f amplifier channels, I am not a proponent of bridging channels for the main speakers, and I believe your mid-bass needs to remain in stereo.  One channel to one speaker for best SQ.  (Subs matter far less as they often sound just fine with 25% distortion anyway.)

Yes, wiring confirmed, IMO.  JL would most likely agree to that.

You can't compare a home system, where the driver response is optimised to the enclosure and the performance is in a room environment, to a car speaker that is designed for IB and meant to be used in a car environment.  Not close enough to the same thing to use as reference.  Go ahead and build your system without a sub, do the damping as I suggested, rattling will be gone and bass response improved..but keep in mind that a sub will not overpower your system unless you want it to.  It's all in the design and setup.  SQ car audio systems need a subwoofer.  Just the overlap in the response curves alone will greatly enhance your midbass sound, and you can limit the amplitude and low end sub response as much as you want to.  Design and setup.