I am considering a Velodyne DD-18...

I've also been trying to decide between the dd18 and dd15. From what I've learned from speaking to people who've had both is that the DD 15 will match the size and speed of your main speakers drivers better than the 18. You should go dual subs if using them for music if it's possible in your budget/room/setup. Avantgarde did a white paper that explained the technical reasons behind that some time ago you may want to reference. My understanding is that if you use only one it tends to cancel out a lot of the cues in the bass information. Remember, digital bass is in TWO channels ! Your sense of space, size of the venue pace and air will improve with the dual setup. You'll have faster and perhaps tighter more musical low end.

Also, some folks have moved from one or two dd 18 's to dd 15 for the above reasons.

Possibly another reason to have 2 subs is that its much easier to dial in a truly flat frequency response in room when you have two to smooth out nulls and gaps. For a multi channel room where you have multiple listening positions, you should also attempt to average out the flattest fr not just for the sweet spot but also the other listeners positions. This is much more doable with 2 subs than just the one Also would like to add that I know of an individual who just sold their DD18's and are moving to the dd15's for better integration with their main speakers for the above reasons, and financial considerations are not a factor. When you look at the specs for both the DD15 and DD18, they both use the same sized magnet motor, both 24 lbs. With the same sized motor, I'd rather have a smaller driver with the same size magnet controling it than a larger, anyday. Now, if the magnet were larger for the DD18 then it may be more of a horserace. I do believe also that there is something to trying not to jump up too large in size with the subwwoofer from your the size drivers in your mains. My experience with the B&W 801Nautilus which in my previous listening room the bass just never integrated well with the rest of the speaker. IMO it was just too large a driver when compared to the mid. The 802N was a better integrated speaker IMO.

Dgarretson , any more info on your IC? I'm using an inexpensive Monster M351 IC. Would I get any advantage to upgrade this?

BTW, the dd 15 is pretty darned great! Got mine Tuesday and it's still breaking in but have been more than happy with the investment. The Eagles on DTS was amazing with Timothy Schmidt's bass notes just providing a rock solid, tuneful foundation. Still working on breaking in and haven't done any torture tests on home theater yet but musically outstanding.

Don't be afraid to experiment with placement I actually got great results with the sub on the left wall firing out in front of the listening position, placement almost 90 degrees from the listening position.

The only thing I wish for is the ability for the SA to adjust phase or help you to determine as I've found that to be an enourmous benefit after getting a flat FR and integrating with your mains.