Though you almost never see it, TL loading is far more critical to the midrange than the low frequencies. The ability of such a design to let go of the notes is unrivaled by almost (I said, almost) any other loudspeaker design.
The medium - larger speakers in the Rega line feature TL loading of the low frequency drive units, and are some of the more underrated and overlooked products out there.
This opened a huge can of worms around here many years ago, but a purist would not consider a Von Schweikert design to be a TL. Instead, they're akin to what Bud Fried introduced as the "Line Tunnel", which is a stuffed, ported loudspeaker. Still, a most competent alignment, offering a lot of the performance of a TL with an even easier (and same low cost and ease of build) to implement design than a typical ported loudspeaker.
My own Fried (8" mid/woofer) A/6 and Fried (10" woofer) subwoofer feature the Line Tunnel. I find the A/6 are not ecliped by most anything built today under $5000.
One thing to keep in mind when dealing with true TL loading of the low frequencies is that almost no one gets it right. The two most common errors are undersizing the line and overstuffing it, the latter being the far more egregious fault. Better to not stuff at all as opposed to overstuffing. The end result being that they actually come across as bass shy and cold, as there is a pretty good sized dip in the mid/upper bass, even if they will play quite deep.
Still, once one has experienced the truly tuneful (and by that, I mean, the ability to discern musical low frequency notes as opposed to the typical and appropriately named "one note bass") low frequency response of a decent TL, it's never forgotten.
