I have a new Infinity Kappa 120.9w subwoofer with switchable impedance (2/4ohm), which is the highest grade Infinity subwoofer currently sold. In my view, it's a fine subwoofer at any price, but at $145 shipped it's a very good value IMO. The sound is very clean and transparent. The upper end extension very nice, making it easy to blend this subwoofer with the front stage. The 0.707 Q is achieved in 1.7 cu ft sealed box running at 2ohms, or approximately 1.15 cu ft running at 4ohms. This is definitely SQ oriented subwoofer, but with xmax of around 17mm, it can also maintain its composure at higher volume levels as well. It doesn't need an uber amplifier. Its rated power handling is at 350watts, but in my car it's running pretty well off 250watt amplifier 2ohms. I am hopping to connect it to 400watt 4ohm amplifier soon.
PS: Unfortunately, I can't comment on References. Given than I am satisfied with the Kappa, I can't see myself running a lesser Infinity sub. However, I doubt that References sound good running off 500wrms, unlike suggested above. Their 12.5mm xmax is easy to exceed with such wattage, and then you have audible distortion. My advise, go for the Kappa...