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?