I know my way around transmission lines and have my own way about designing them. I used to use a modified Martin J. King Mathcad file where I added a section for the vehicle's chamber and modeled my old integra hatch with an Acoustic Elegance AV12 in it perfectly. I matched the meter's response accurately. It also did a 141ish at 22hz and could do a minor hair trick with windows up and sunroof open (cheating I guess). It could also float a piece of foam like you would find in a toolbox in the sunroof area at 22hz. It was -3dB at 17hz and 75hz and was ruler flat (within .1dB) across the 24-70 section but did have a +3dB rise right at 22hz but was back at 0 at 20 and 24.
Critically and over damped drivers are actually really great for getting down low with authority and playing smooth and flat across the band in a transmission line. A strong motor and suspension is needed to keep the driver under control. My current setup is a self loading transmission line (seen here:
http://oi35.tinypic.com/14t96b8.jpg )for two True Bass Solution ultra 8s. They're over damped, high Fs drivers yet they play down to 25hz with authority in my trunk Nissan 240sx and have quite a bit of output for only getting about 100wrms each (louder, lower, more flat, and way better transients than two Boston 12s sealed in a Focus hatch getting 300wrms each). I'm not trying to sound like I know everything because I don't, but from my experience with various drivers in various transmission line styles, I very much prefer the over damped and critically damped drivers for flat and extended low end response. If I use a higher Q driver, I almost always have to taper with about a 2:1 ratio and add stuffing to help dampen the response. While I agree with what you said for bass reflex and bandpass enclosures, I think it's a little off for transmission lines. The LMS isn't a bad option but that **** is way over priced for what it is though I have enjoyed the couple I was fortunate enough to hear.
Nice vid! If your line is designed around your sub's Fs and SD only, then it's going to be very compromised and not really have the best results. Sure it will work but it won't be as effective as it could. Take a look at the Alignment Tables PDF from this site. It gives a much better way to calculate line area that incorporated motor strength and other factors that are critical for getting the right numbers. Also, the Fs is not a good number to use but rather the Fc of the sub in a sealed box with equivalent air space that is in the line itself. That will get you much better results both in output, low end extension, and sound quality. My little 8s are no joke with just 100w each and when I briefly had them on 600w each, you couldn't smack the smile off of my face haha! I'll keep you updated and do the same with me. But definitely give MJK's page a read and see if it sheds some light on your situation. That man knows his ****!!
Edit: oops forgot the link...
Quarter Wavelength Loudspeaker Design
Click on theory then the Alignment tables.