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.
The issues of room modes will remain. There are basically two ways to deal with them:
1. (re)positioning speakers and/or listener to avoid the worst spots
2. acoustic room treatment to tame them

The EQ on the DD-subs is an excellent one that will help in any room but it cannot fill a (bottomless) null.

BTW, the DD-15 should be OK in that room, if you treat it right. ;-)

Velodyne has a sub selection wizard on their web site based on room volume. I would accept their advice.

I use an M&K 12" sub in a 17'x17'x12' room, so the DD-15 should be fine.
I have a DD-15 in my basement and it's set to 6 or 7.
The DD-18 is most likely more than you would ever need.

I agree with the other posts that state the dd-15 will be plenty adequate.
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.
I have a DD-15 and it is very powerful. I tried the DD-12 and it was very impressive as well. I went with the 15 just in case. Everyone that listens says it's the best sub they have ever heard. Even my friend that has two Aerial subs and a DD-12 for his low frequency effects says it's the best.
There are also people that claim bass can be cancelled with two subs because some bass is recorded out of phase. I think there is always a trade-off. If I were going to use two subs, I would go with the DD-12s.
I don't believe in the speed issue because to produce the same amount of bass a smaller sub has to move further. Therefore it cannot be faster unless it produces less bass. It could actually be slower when producing the same bass as a larger sub. A smaller sub also has to work harder. The only real advantage I see in size is a smaller footprint and less cost.
Velodyne tech support will tell you that the larger the DD, the better the performance, due to the larger, lower-excursion cone placing an easier load on the 1250W plate amp common to all models. I'm using a DD-15 in a similar room sized 16x16x9. If the Joseph's stated bass extension to 32Hz +/-2db is true, then you should be able to crossover at a low 30Hz or 35hz, where the xover is unobtrusive and bass is very omnidirectional. In this application one sub should be fine.
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.
Magnet size in a sealed enclosure should actually be smaller. But the 15 is a great sub on doubt.
Thanks, everyone, for your responses. The dd-15 appears to have the edge. Velodyne's website suggests that either is acceptable. Anyone have anopinion on their respective musicality? I should have mentioned at the beginning that the application is strictly two channel music.
Well, I've found the DD-15 sounds clean and powerful in my 2CH set-up with Merlin VSM-MXs and a BAT VK75SE. If the DD creates excessive floor resonance, try elevating it on an Aurelex foam platform. If you have good electronics, you'll prefer using the DD in LPF (bass augementation) mode, rather than HPF xover back to the main speakers.
Thanks, Dgarretson. In my current setup I run the B output from my Lamm LL2 out to a Sunfire Superjunior sub. The A output goes through an Art Audio Jota and from there out to Joseph 25Rm SigII speakers. The speakers claim +-2Db down to 32Hz but I measure a roll off much greater than that from about 40Hz and below. The Superjunior flattens that out down to 30Hz but can't keep it up in the sub-30 range and that's where I hope I can buld up some response.

Is placement in a corner/next to a wall critical?
Put the sub next to one of the speakers and use the amp outputs to the sub inputs.
Very nice system. You might compare both A & B options. Personally I prefer line-level inputs from pre to sub, even with tubes in the main amp. I think interconnects make a big difference in the quality of bass. I use a long DIY copper ribbon that's very good with LF extension and dynamics, and cheap to build. Room placement is less critical with the DD than with other subs. The spectrum analyzer, 8-band digital EQ, and granular phase settings, can compensate for most room anomalies. There is enough headroom in the sub amp to stack several bands of the equalizer on top of each other if necessary. Corner placement usually provides the most reinforcement, but not necessarily the cleanest bass. If you buy used, update to 2.0 control software via the Vel web site.
no one gonna mention the JL Labs, I mean, JL Audio f113?

it seems to have set a new standard in home subwoofers and is a bit cheaper i think.
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.