Hi there!
Interesting thread as I am in a similar postion, living in a first floor flat with neigbours below (but none to the sides or above

) and am thinking of getting a sub to complement my Wharferdale Diamond 5.0 set up.
I'm not after room shaking effects or thumping bass, just want see (hear?) what I'm missing below the lower limits or the 9.1's and to take some of the work away from them to hopefully get a better sound.
I'm slightly confused though, assuming two subs were both set up correcly (using SPL or whatever) would a larger sub (e.g. bk xls200) transmit more annoying bass to the neighbours than a smaller one (e.g. bk gemini), or would this be offset but other factors such a less 'boominess' and better control?

Or does a bigger sub have more bass extension at lower frequencies which is even more annoying?

I would probably use the sub for everything (music, games and movies).
Cheers
Chris
P.S. When I got my amp and speakers I let the neighbours (young couple) below know and asked them to let me know if they could hear it. They were fine and said not to worry, as long as I wasn't playing loud music late at night. Anyway, they've never complained.