+1 @erik_squires on the Transmission Line being competition to the usual front, rear or bottom ports. All extend bass. I learned a lot looking at PMC speakers about Transmission Line. They are touted as being tighter, less bloated bass compared to some ports. Additionally, they allow placement of the speakers closer to the rear wall. PMC seems to specialize in transmission Line bass. I am a horn dummy though.
Differences Between Folded Horn Speakers and Transmission Line Speakers
I've been looking at various DIY speaker builds and came across a folder horn speaker and I've also seen transmission line speakers. I've tried to google the differences in the two, as they look similar, but I suspect that there are differences. The only visual difference that I've noticed (I've only seen one folded horn, so the comparison pool is very small) is that the folded horn seem to have a larger opening than the transmission line speakers. Maybe it's just the one speaker that I saw, so I don't know that would always be the case. I'm hoping someone on the forum is much more knowledgeable about these things than I am.