-"think" the old Karlson box with stock port probably has 3-6dB gain over a reflex with back chamber and Z minimum "wquivalent" (~50Hz/~95-100l) plus distortion may be lower in places but 1951 K15 is bulky at 7.8cu.ft. external.

with lower qts drivers it might be possible to get an alignment where back cahmber volume is dropped and front chamber's mass loading flattens the passband while further reducing distortion

the 1965 X15 15" dirver Karlson at 4.2cu.ft. external bulk seems like could be a good bass guitar box or maybe midbass for PA - seems to play awfully strong w/o too much cone movement within its usable passband - suspect by 50Hz that has pooped out back to reflex equivalency - or maybe worse due to only ~2.2cuu.ft. rear chamber.

Karlson seem tricky - changing wing profile just a bit makes a big voicing difference when loaded with coax.

Heres K15 vs a reflex having calculated 54Hz f3 graph

All the best!

Wayne