The Karlson isn't a folded horn, it is its own unique design, closer to a ported. vented cab. The designers (John Karlson and Wayne Green) applied 1950s microwave antenna designs to audio wavelengths.
I haven't tried it for bass guitar use, just LOUD music, and it delivers the goods in the low-freq department, effortlessly.
The woodworking for these cabs is fairly difficult - I was lucky enough to find a pair of these cabs at the local town dump / recycling center, in the "swap" section. So someone else did all the hard work about 40-50 years ago. The design goes against all of the common wisdom, no acoustic "stuffing" inside the cab, in fact it is interlally coated with some form of hard resin or epoxy. The venting is a series of long narrow slots, and the front baffle has a strange log-curve, and the speaker itself is mounted on a slanted bafle behind this.
A picture is worth about ten thousand words in this case...
Original "Karlson" posting starts here:
http://diytube.com/phpBB2/viewtopic.php?t=781
/ed B