I Need Help Picking a Sub

Hello all. Right now i have put together of a somewhat decent theater system in my small living room. I need help with a sub woofer because i dont know what to buy. I am giving myself a cap of $1400 for something used on audiogon. I have Martin Logan ascent front and center speakers. I have a sunfire pre and 5 channel amp all of which are balanced connections. i am looking at the revelB15 sub or perhaps a velodyne DD series. because my space is tiny i was thinking nothing more than a 8" woofer but someone told me to stick with nothing lower than 10" for clarity. i could use some help from those audio pros out there.