Dang, old post. I've been running a DD18 since about 2006. It's still going strong. The equalizer can be a factor but I agree with the more recent wisdom that multiple subs are better than one. Equalizers are for small problems. You can fix a bad frequency response peak by cutting 15db but it never sounds right. You need to have a pretty good in-room response and finish it off by making small eq adjustments.
I am considering a Velodyne DD-18...
...to complement my Joseph RM25 siII speakers. When I went looking for speakers with more bottom end the dealer who sold me the Josephs said that in my room, 16'X13'X9', I'd have trouble with resonations. He suggested that a sub that could be tuned to the room would give better results. After much reading, I'm gravitating toward the dd-18 but I have heard it said that a smaller room may prefer the dd-15. I assume that the dd-18 will better reproduce the very, very bottom end and that the equalizer eliminates the issue with the room size. Anyone have opinions on the above? Thanks for any replies.