I have for the longest wanted to compare several 12 inch woofers and last winter i had the chance to purchase one used (i couldnt afford the $600+ new price). I didn't get a chance to switch out my Heavyweight for it until this past summer and drove around with it all this semester to see how I liked it.
The subs are definitely similar in sound, and I can see now why so many struggled to give a fair comparison. One thing i noticed is, in the same box, it didn't hit the lows as well but did hit the highs better, where as on the higher bass notes the Heavyweight basically poops out and you feel nothing. It didn't move nearly as much and required a less power than the RD (i gave it around 2500wrms) which was a plus as far as helping save my electrical system. Also cosmetically the DD looks WAY better than the RD, especially with the Carbon Fiber dust cap and i personally love. Besides that they were basically the same imo...
Due to my preferences i switched back to the RD. In the end it had much better SQ and got a lot louder especially in the low bass notes. I think the DD is a much better choice for anyone that wants a speaker that plays a wider range of notes or has less power to offer. Definitely not a bad choice though, just rather pricey for a 12" subwoofer.
Sound Quality: 8/10
Power handling: 9/10
Low Frequency Extension: 9/10
Build Quality: 10/10
If you're not passing anyone, get out of the passing lane.