All of the HDC3's have the same motor, if I'm not mistaken. And the 2510 is no pushover. My single 2510 did a 141.0 on ~ 800w, my pair of HDC3 10's on an AQ2200@1ohm are doing 143.7. I know there are more factors in play, but doubled the cone area and doubled the power and got +2.7db. The AQ is only going to kill the 2500 because it can take more power, but 1500 to each and the HDC3 is gonna catch hell from the 2500. Put 3k on the AQ and of course it's going to kill the other two because they can't take it.
So how do you do a fair comparison, really? Wouldn't any quality sub of the same size and comparable power handling be pretty **** close? Put all 3 in the same box on the same power and I'll bet they're all within a couple tenths. It would probably come down to tuning and whichever sub performs best at that tuning. They all seem to want ~ 1.5 cubes, but DD wants a 37hz tune and AQ wants 40. Who knows where Cadence wants to be.