Velodyne DD-series subwoofer

I am currently deciding between a Velodyne DD-15 or Velodyne DD-18 subwoofer for my home theater system. My room size, not yet confirmed, will likely be at least 15 x 20 feet and possibly larger if there is an adjoining kitchen and/or dining area (although there may be no adjoining area at all, so the room may only be that size or even a bit smaller or larger). The room height would be about 9-10 feet. I have a very good deal on all the Velodyne DD subwoofers, so the price difference between the subs are insignificant.

Music/HT is about 60/40.

Is there any reason NOT to get the 18" subwoofer? Is it possible it would be too large for my room? What is the smallest room for which the 18" sub should be used? How would two 15" subs compare to one 18" sub? Are two 18" subs feasible, at least for future upgrades?

I live in an apartment- it it possible to use an 18" sub without disturbing the neighbors? How about the 15"?

My equipment currently includes B&W804s and B&W HTM1, and will likely be using a Bel Canto 3-channel amp to drive them. pre/pro/surround amp is Yamaha RX-V1500, with B&W DS7 surrounds probably.

You'll bother your neighbors with either one!
No slouch -

There is NOTHING like an 18" sub for 2 channel or theater. The DD-18 will keep up with any sub in the world for 2 channel, especially the Rel. I personally feel the DD-18 smokes the Rel subs, and feel if everyone could afford the DD-15/18s (they are ridiculously expensive) most would change their minds as well. Many are still hung up on the F-1800s and HGS-18s, but the DD-18s are a completely different animal.

As for the 15/18 disturbing folks, they BOTH will!

Great subs, the determining factor should be real estate. How much floor space do you want to dedicate. I have a room similar in size and have the DD-12 and it does just fine. If anything I might recommend, since the price is right for you, get 2)DD-12s you would get much better imaging and distribution. Either way you will love your DD Velodyne.
If money and the size of the sub are not an issue then just go with the DD-18. If size is an issue then go with the DD-15, you will probably only be giving up a few db's of headroom and a couple hz of extension. I have a DD-15 in a 21 x 15' room with and 11' ceiling and I have more than enough quality and quantity of bass to satisfy me for music and HT. Dollar for dollar they are a much better value than a REL.
If you are in an apartment then decouple the sub from the floor to prevent shaking your neighbors physically. Try some Aurio's under the sub, and mass load the top with some lead shot (25lbs #6)

DD15 or DD18? Awesome problem to have, I basically couldn't use my sub in an apartment before decoupling the sub. Invite your neighbors over during movie time..
Expect the police soon; thereafter. Maybe you have a movie they haven't seen?--This is a joke,right?????
I use an Auralex Gramma under my sub and it does an excellent job of decoupling the sub from the floor. It only costs $60, too.
I also suggest the Auralex Gramma. And some common sense regarding volume.
Sorry, I realize I sound like the newbie I am now - I've never really experienced a sub in someone's home before.

I can see from the responses that 15" is probably too much for me - more specifically for my neighbors. Walls are thick concrete here, and apartments are very well separated. I listen to music and watch movies at relatively loud volumes (I don't blast them, but at the same time i never think about the volume in terms of worrying someone might hear) on 6" floorstanders day and night, and my neighbors can't hear a thing.

I wasn't really going for decibels but rather for good bass extension at low volume, and I thought a larger subwoofer might give me that. I read other people's postings about how much a subwoofer can radically transform their system, so I wanted to go all the way and not feel I was missing bass at the bottom.

So I should go for two 12" subwoofers? Would I be sacrificing much separation of low bass notes and sounds? If so, how much, qualitatively speaking?
at least you have concrete walls which will help! If you like it loud, then you might consider positioning the sub behind your couch (my 18" is now set that way and floated) and get a ButtKicker to get the impact of a loud sub plus the sound of a loud sub.

I would still do at least a 15 if you can afford it, you won't be there forever.. If that is not enough then buy a second one.
thanks for the postings on the Auralex Gramma- I just checked it out...sounds like a great idea if it's a sound one...(pun intended)..:-)

i just read this article on it: