i wuold say dd. it sounds like you werent giving them enough power. you really need more power for those than 700 watts. i think that the 3500's are one of the most musical sounding woofers i have heard to this day. it doesnt only play deep, or only high, it is a very well balanced speaker.
oh I gave it some power, this was just when i did a direct swap from the rl-p to DD. The amp put out maximum power at 1 ohm(concept audio 1200D) and the DD was dual 4 ohm. But later I put a MTX 1000D on the DD.