Small sub

if you can fit a jl audio fantum 112, it is the best sub out ther in what I cansider a small size. Trouble is a sub has to move air which by definition means some size. JL cheats a little since they have one of the longest throughs on the market. I also like velodyne alot. Personally I don't do small subs I never found one that wasn't to much of a compromise. the f112 is the smallest I would go.

Good point for your sub you need to consider room volume H x W x D = cu ft. This is important you need a big enought sub to pressureise the room or it will be point less, actually you shouldn't determine the sub size the room should. Also you need a big enought cone to get the low notes 21 hrtz is usally good watch the specs on small subs they just never sound deep to me ot the ones that get that low have big distortion at volume. Last there is a guy you may want to google craig's subs he seems to of dedicated his life to nothing but testing subs.