You can tune reflex boxes much deeper than Fts, and, in fact, if Qts is less than 0.375, it probably should be tuned lower than Fts. System response is determined by overall system specs.

Generally speaking, bass-reflex systems with drivers having Qts less than 0.375 will be tuned higher than Fts and their f3 will also be higher than Fts. Those with Qts greater than 0.375 will be tuned lower than Fts and f3 will be lower than Fts. When you get above Qts of 0.7, response will be underdamped. These are generalized cases, and there are plenty of other ways to tune the system. But this gives you a good idea of what to expect.

Martin or one of the other TQLP builders should probably address the subject of transmission lines, because they may have some special circumstances I have not considered. I would expect that the primary resonance mode is tuned pretty much the same way the bass-reflex cabinet is, and so the tuning characteristics described above probably apply. But there are secondary resonances at harmonics of the fundamental in a transmission line that are dealt with in a TQWP. So this may make other conditions apply.

Wayne