I agree with you. But I'm running about 1400 or so watts, which, I would think, would be more optimal for the 3500. I was just surprised at how much of a difference it was. It also hits a wider range of notes as well.
err the 3500 can take 2500wrms, i run a 3518, and my buddy runs a aq hdc315 both great subs the dd will like a bigger box than the aq both will take 2.5kw but over that (besides 3-4k burps) id go w/ the 9500 or hdc3
My 3512 didn't start to really shine until it was given around 2000rms but even with that power I highly doubt it would compete with the HDC3. In my comparison I have always felt that the DD sounded a lot better but I believe the deciding factor in your case in term's of the SQ is the fact you aren't using the ideal box and tuning for the DD and the HDC3 just happened to function better in that particular box.