I have the HC 15" hooked up to a JBL 1200.1, and even in my big expedition, it knocks. Sounds better than any other subs I owned even though they were low end. I'd say 50/50 sq and spl. Gets just as loud and sounds cleaner than the 3 15" rockfords i had, and takes up a 3rd of the space. Definitely an awesome sub if you can find one somewhere, i see lots up on ebay.
P.S. Impressed with the JBL 1200.1 as well, paid $200 including shipping, worth twice that, love it.