That was going to be my suggestion, you beat me to it!
I have a pair of CS-99 woofers that I'm looking at, very nice cast frame, big, heavy magnet structure.
Very nice, but they really don't compare to the HPM series woofers. The CS 15-incher weighs about 14-15 lbs. The 12-inch HPM woofer weighs 20-22 lbs. My back guy would frown on me playing with the HPM 15-incher!
The CS-99, like the HPM's, was a ported design. The CS-99A was an "infinite baffle" (sealed box) design.
Should work, though.
I want to know how they sound with the BIG HPM-150 box and complement of drivers.
GOOD LUCK!!
pete