Since the O.P. asked specifically about a small sub in a 2-channel set up I'd like to recommend the Dynaudio 9S which is almost half the size of the
SVS SB12-NSD which it replaced in my system. The 9S, being a pro studio sub, has the big advantage for my system in having the option of all balanced XLR inputs and outputs which allows me to operate my whole stereo in fully balanced mode for which my gear is optimized.
It's also a fantastic sub, easily exceeding the SB12-NSD which, by comparison, sounds rather bloated and sluggish.
It's also a fantastic sub, easily exceeding the SB12-NSD which, by comparison, sounds rather bloated and sluggish.

