Seriously though mate, you will find that the smaller your front pair are the more difficult it is to get a sub that can do that sort of range, be good at home cinema and music too.
I have a pair of Mordaunt Short MS45Ti's on 14 inch stands and my sub a yammy YST-SW80 (which is crap at home cinema by the way) is set at 50hz the lowest it can go and 180deg out of phase it was a doddle to set up realy but that's because i have large front speakers which do a lot of the low Hz donky work, (tell me if i'm wrong here mark) but the lower down the Hz scale the sub goes the more out of phase it will become, so if you're needing a sub to kick in at 80 or 90Hz say, but extend down to 20Hz it's going to be a tricky job to get a good compromise with regards to a phase setting so choose a sub which has a variable phase setting between 0 and 180 and you'll be covered and much happier with the results