Have no knowledge of karlson cabs - can offer 15" woofer suggestions that work in bass reflex cabs as subs.

JBL 2235 is great. Only available second hand though as its discontinued. Adire Tumult is also good quality stuff although you need over 2KW to hit 120db due to its very low efficiency. Don't use the Tempest. I think TC Sounds also make some nice 15"ers. Again, no idea weather or not these are suited to karlson which I assume is a horn.

Adrian