N802 + Velodyne DD-18???

IMHO, if you need a sub with N802s, either you're not using the right amp or you're using the N802s in an application where you would be better suited to use other speakers with a sub.

Again, just my opinion. I could be wrong. My wife often tells me so.

How big is your room?

Everyone needs a sub. I have the DD-15 and it's great.

The 802's only go down solid into the 30hz range.. a subwoofer is an excellent addition, plus this lets you put the 802's where they sound best and the sub where it sounds best! I used to own an F1800XR Velodyne 18" sub and went down to a DD15 and was disappointed (it hit harder but but didn't work as well with music)..... I would definitely spring for the DD18 unless you think later you could go for a Dual DD15 sub setup which really helps out the acoustics.

Otherwise a JL Fathom F13 works quite amazing... compared to both DD15 and DD18, except the room correction is better on the Velodyne.

Oh yeah drop a 25lb bag of #7 lead shot onto of the velodyne and be prepared to be amazed. Isolating the sub on Aurio Pro's, Sistrum stands, or Auralex sub isolator will also enhance your experience.