Ime TL’s are very good in the midrange, as there is essentially zero reflection back into the cone. That may well be their biggest advantage.
As for where the terminus (line opening or vent) is located relative to the woofer I’m sure that makes a difference but haven’t really analyzed it. My instinct would be to spread them apart as far as is reasonably possible in as many planes as you can, in pursuit of modal smoothing. Like if the woofer is up high on the front, and the terminus is down low on the back, if you toe the speakers in, now the woofer and terminus are displaced relative to one another in all three dimensions.
But I cannot reliably say that’s the best strategy for choosing where the terminus winds up - other considerations that I’m not taking into account may dominate. For instance the internal geometry may matter more.
I remember delivering a pair of my rear-ported speakers to the home of a customer who was replacing transmission lines that had the terminus on the front at the bottom. One of the first things he commented on was that the bass was smoother with my fairly low-tuned rear-ported box. Whether that was because of the terminus location versus my port location, or my speaker’s freedom from the half-wavelength bump and/or one-wavelength notch, I do not know. I was surprised that my speaker was competitive with the transmission line in the bass region.
Duke