Transmission line enclosures are not correctly designed by taking the Sd and Fs of the driver and making them your line length and cross sectional area. It will work but it won't really be optimal.
T-lines are not always more efficient than ported or bandpass systems and many times they transfer gain for bandwidth.
The sound is just like Brian described as the driver can sound very natural and the roll off is much more shallow than compared to ported enclosures. The electrical impedance also should stay pretty flat and the natural peak around resonance should either be lowered or about the same as the nominal frequency. The extra air mass in the line will act like a mass on the drivers and it will substantially lower the resonance point of the system.
For instance, my AV12 has an Fs of 24hz and 76in^2 of cone area. I designed a line with only a 35hz line length and only 42in^2 of port. The sound is phenomenal but efficiency is much worse than the ported enclosure I had that was 2.2 cubes tuned to 32hz. I had to greatly increase the gain to get respectable output but when I did, I got a very smooth response that got very low and blended with my front speakers effortlessly. It sounded much better than the sealed enclosure I had it in and could get much lower.
This is the measured response I got in car at low volumes (high volumes not possible due to using only a 100wrms amp):
Above 60hz, I'm out of the main boost from the transfer function. With the mids playing, the response is boosted and is pretty **** flat. I don't have those measurements on my PC though.