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
Although I like the idea of large LF drivers, I do not think that the 12 inch Velodyne can be described as a "toy". If space or budget constraints must the recognized the 12 inch units are a good choice.

About the 3 step equalization process...The second SW may null out a room resonance that the first SW saw, and corrected. So in the end that correction by the first SW is not needed, and the second-time-around equalization gets it right.
Eldartford -

Your explanation of the set-up of dual subs makes sense. However, isn't there then a chance that by making a second adjustment to sub #1, it could change a resonance that sub #2 originally saw and compensated for, but is now not needed. It almost seems like chicken and egg to me. Am I missing something?

I agree with your opinion of 12" subs. IYHO, forgetting space considerations, am I right in assuming you would opt for one DD-18 over two DD-12's?

Thanks.
Kenl....I suppose you could go back and forth forever adjusting the two Subwoofers. The suggestion from the Velodyne guy seems reasonable.

I am glad that I don't have to decide between two 12 inch and one 18 inch. I guess I would go with the two 12 inch.

FWIW, my subwoofer system consists of three 15 inch and three 12 inch drivers, in huge enclosures embedded in the wall.
The guy from Velodyne is old school. He is a hoot to talk with. He rminds me of Tom Port from better records. I was also thinking about two DD-15's. For me it's just overkill. My DD-15 is set to 4.
Just so you Know I run a Velodyne F1800X behind my couch in a HT setup and now I don't need bass shakers in the couch because it moves the couch plus it doesn't have to be quite as loud, you must crossover at 80hz or below becuase it is in the back of the room. So under my screen is now uncluttered! For movies go for the single 18", if it was music dual small subs but the Electronics will take care of the room problems.