Hi Neopsp,
A driver used in a transmission line will have the same response as a driver used in an infinite baffle arangement. The free-air f3 of the driver will be the same in the TL enclosure. The theory is that the "transmission line" is of infinite length and therefore places no load on the rear of the driver which is isolated from the listening space. The driver's front radiation is all that is used. Pratically, the TL is stuffed with absorbing material to absorb all of the back radiation, and just "looks" like it's infinitely long.
Drivers designed for this application need to have a stiff suspension to support the cone and voicecoil and extra mass to get the f3 low enough to be useful for hi-fidelity. That forces the motor to be very powerful to drive the stiff suspension and high mass. High mass tends toward high Qms, powerful motors tend toward low Qts and Qes. If you go in the opposite direction with the Q values, you'll get higher f3's and/or lower efficiencies.
Hope that helps.
Rodd Yamashita