I squeezed an Arc Audio KS 125.2 Mini under my passenger seat, it powers my passenger foot well sub

 

Well my planar is used as a pure midrange 200hz to 5khz and then a 3/4" takes over and is omnidirectional to 18khz. I know that sounds scary 200hz and all but I've tested them vs. an MTM of pro audio midranges and the planar got much lower distortion, especially at 200hz all the way to 106db. These new generation BGs are the best midrange money can buy.

I saw the UL12 with the busted leads, but if it's a common thing it might have been some other one. I've been interested in them ever since Pat tested one in CarAudio mag. It's basically the Klippel king. You should really boost the bottom by a lot though, it has less low end sensitivity that most 8" subs. There is a reason why it can take so much power the resonant frequency and Q are really high.

 

I here what your saying about the Q and resonant freq but sometimes what it says on paper can be a bit deceiving. I have been surprised by a few subs in the past because on paper they look pretty basic (jl audio 13W7) once you throw power behind it the specs go out the window.
The trunk is so small in the S that a free air setup may work, but I have never been a fan of the actual sound of that setup. I may have to play with different enclosures and even try a free air and see what I get sound wise. I'll call my buddy up in new York and see when is next trip to Florida, he is the only person that I can work with and bounce different ideas off of

Apogee used a planar as a mid in there high end home speakers to great effect, I wish I still had mine they are very detailed and accurate but lack the energy that a cone mid has but I guess you can make some of that up with the mid woofer in the door, the hard part is the crossover point.
Why didn't you use another planar for highs?
The TC sounds planar I have are smaller than those bg's and are a lot better above 3k than a lot of tweeters I have heard and don't have that dip at 5k a lot of other planars have. I picked them up with plans to install in my last car but never got around to it

 

The 13W7 checks out on paper, it has great Klippel performance and great specs, that one should work fine. FS and Qts are halved. There is no shorting ring whatsoever and the linear throw is about 30% lower so the motor and suspension aren't as good but at least it has applications it would work in. I thought about using 3 of them but I could never power them even in IB from a single alternator. To me IB is a necessity in a car if you want to get even close to an ideal Q and have decent output at the same time. I would personally go for the 13W7 and well there is no use for the UL12 anywhere. Maybe one day someone will design the same sub but drop the voicecoil mass and design it to extend low from less power. You can no longer market it to car audio kids that it can "handle" 3kw but it will work better at 500w.

There isn't a single planar on the market that has the dispersion and output of the 3/4" Scan Illuminator. Like you said, dispersion is a big problem up top. There is a 1" AMT tweeter that is decent to 12khz but that's bellow par dispersion and then only 20w power handling compared to 120w IEC on the Scan. I only use new technologies when they beat existing ones.

 

The planars I have play from 900 to 20khz at 60 watts but cut higher they can hold more, I forgot the name of the guy tested them along side a few other planars and he rated them above the others. The 5k dip and 12k spike in other planars was non existent in these, they played fairly flat from 3k to about 17.5 then started to drop off. I only know of 1 other set out there and tried tracking the owner down but came up empty.
I have to disagree with you on the UL12, I sat in a customers car for about 15-20 minutes playing different kinds of music and was really impressed with the output. His setup had 1 in a sealed box around 1.75 cft and it played hard, tight and very musical. At the time i was running a custom built OZ audio power 15" with a sundown 3k, for the most part it was loud as hell and played pretty damn good, the UL was not nearly as loud but was more musical, bass notes had better detail.
The jl 13w7 on paper looks very good but there are other drivers with similar specs but dont perform nearly as well, once the jl is powered up correctly it is a different ball game, there aren't many car audio drivers that can compare with the brute force and sound quality.

 

Is the area below the seat good enough to let the amp breathe ? Its a pretty tight fit in terms of height but there is obviously room on the sides of the amp and free airflow from the footwell

 

If its a Class D amp then it should be fine.

 

Depending on the size of your amp, there is some room to the left of the spare tire on the shelf the tire mounts to. Thats where one of the 2 optional OEM CD changers mount. ( the other mounts in the center storage box) Pull the trim off that covers the spare and see if its big enough for your amp.

 

or here. i love mine here. doesnt get in the way whatsoever!

 

You could always mount them on the desk lid... I had mine up there years ago, so to all the nay sayers that say it can't be done... it can but I will warn you, it does make the deck lid heavy :