15" vs 18" subwoofer - which to buy?


If price and room space/aesthetics were not a big issue, is there any reason to get a 15" subwoofer over an 18" one? My main issue is not disturbing the neighbors too much. I'm confused about the difference between a 15" and 18" subwoofer. I want to get the best sound quality possible for low-to-medium volume use.

I've heard that an 18" subwoofer can be played at low decibels and provide good bass resolution and fullness of sound, while to achieve the same volume with a 15" subwoofer, the power has to be much higher. So for any given sound volume, the main difference is in distortion- and higher distortion sound may be more obtrusive. Is this true? it seems counterintuitive that an 18" subwoofer could be better for neighbors than a 15" subwoofer.

However, I compared a B&W800 (12") vs B&W850 (15"). I listened from the other side of the store wall (not as thick as my apt), and at minimum levels which I found satisfying for HT, I found the 12" more obvious and 'boomy' sounding while the 18" produced a more subtle (though actually more powerful), lower frequency vibration, like a very low-level rumbling background earthquake. The 15" subwoofer did seem more of a disturbance because of it's 'obvious' sound. However, I worry that an 18" won't even start producing quality sound until a certain volume that was much higher than a 15", and consequently potentially more disruptive for neighbors. Is this true? What is the relationship between subwoofer size, low sound volume, and subjective listening experience?

Unfortunately, it will be difficult to get an opportunity to hear both the DD-15 and DD-18 which I am considering, so I hope someone can help out here.
no_slouch
Sean...What are we looking for? I thought it was a variation of sensitivity (dB/volt or dB/watt) as a function of SPL, particularly evidence of a reduction at low SPL. That's what I looked for.
Thanks everyone for your comments.

I put in an order for a black DD-18 subwoofer. Originally I thought I wanted the maple, but I really liked the gloss finish on the black.

In total, I listened to the DD-12, DD-18, BagEnd Infra18, Rel Stadium III, B&W800 and 850. My impression was that an 18" subwoofer could be played well at low volume. I had never heard a home subwoofer before, and was amazed at what it added to movies and HT, hence I decided to get the biggest one I could. Although the Velodyne retail salesperson and various other people on this forum and elsewhere have assured me that the smaller 12" and 15" subwoofers should work about as well in smaller rooms (they said that room size should be my only consideration), I didn't have the chance to test this. In the small to mid-sized showrooms (250-400 sq feet?), and sitting at a close distance to the subwoofers, I found it hard to believe any number of 12" subwoofers could ever match an 18" subwoofer. The 12" ones didn't seem to have authoritative impact, and in fact I didn't really like the 'punchy' bass they put out- I see why someone might call them a toy, no offense intended to anybody. Is it a non-contested fact that multiple 12" subwoofers could match or best a single 18" subwoofer in terms of quality of low bass? It seemed so hard to believe in the store.

Also, a reviewer for a hifi magazine was in the store that afternoon, and I got to listen in for his review of the 1812 subwoofer for about 45 minutes of home theater, and I decided I wanted to get as close to what i heard there as possible

The DD-18 will arrive in about 2-3 weeks at my apartment, I'll try and post a follow-up around then.
Is it a non-contested fact that multiple 12" subwoofers could match or best a single 18" subwoofer in terms of quality of low bass?
In theory you can do it. In practise it is VERY difficult to simulate a single point source (the 18") with many 12".
You'll be happy with yr DD-18; it's a very good product.
Gregm..."VERY difficult", perhaps, but to simulate a point source to the left or the right with a center speaker is downright impossible. I agree that the Velodyne 18 incher is a good product and will no doubt work well.
No slouch,

I too listened to a DD-12 at a local dealer over the weekend, and while it did a credible job in a moderately large home theater room, it didn't provide the tactile shudder of an 18" sub. Like you, I'm going to go with the DD-18. Still thinking about 2 of them? I suppose you won't know until yours arrives.