In my opinion the C92-6 is the only Accuton unit I see on Madisound's website that's a reasonably good candidate for transmission line loading.
I have built roughly 50 different transmission line designs, though many were experimental only. Here is what I learned:
Go by the physical line length, measured down the center of the line. Ignore anything you read about the speed of sound slowing down in a fibrous tangle.
Bass extension to the 1/4 wavelength frequency is not easy to get. Maximum reinforcement will be at the 1/2 wavelength frequency. Up at 1 wavelength, we have a problem: The output from the end of the line is out-of-phase, so it will cancel the frontwave energy and you'll get a notch in the frequency response. Now notches are not nearly as objectionable as peaks, so one option is to try to ignore it. But once you know it's there, you'll probably hear it as a leanness in the midbass.
By increasing the stuffing in the line, you attenuate the backwave energy that emerges 180 degrees out-of-phase at the 1 wavelength frequency. But, you are also attenuating the reinforcing backwave energy that would have helped out at lower frequencies. So, it's a trade-off - in exchange for smoother midbass, you give up deep bass.
In my opinion the solution is to start out with too much deep bass to begin with. So I'd suggest high Qts woofers for transmission lines, so that when you overstuff the line to smooth out the midbass dip you're also bringing the deep bass back down to where it sounds balanced. I'd go for a Qts of .6 to .9, which is higher than any of the Accutons but the C92-6 comes the closest.
Finally, you can smooth things a bit more by locating the woofer 1/3 of the way down the line instead of at the very end of the line. Or, you can use two woofers with the midpoint between them 1/3 of the way down the line. You get maximum power handling and reinforcement if the woofer is at the far end of the line, so once again it's a tradeoff between smoothness and bass extension.
In my commercial efforts I don't use transmisson lines because the woofers that have the qualities I place high priority on do not work very well in transmission lines. The woofers I like are better suited to vented enclosures, so that's what I use.
Best of luck to you,