All Kustom 3 speaker cabinets used 16 ohm drivers for a parallel load of 5.3 ohms.
Ted Weber makes a nice 15" Bass driver with a 60 oz mag, cloth surround ( even better than the CTS driver) and is rated for 175 watts and can be had in the needed 16 ohm version.
There 135 bucks each which is not bad!
The weber stuff is great....have used them for many years on my pro backline rental rigs...replaced lower wattage Jensons, Blue Dog 15 and 30s for my Vox AC30s, etc...great folks to work with and you'll be very pleased with any and all of their drivers.