Subwoofer choice, Sub to pair with Sonus Faber

I have a pair of Sonus Faber Concertos in my bonus room that I will be relocating to my loft so that I can do some more serious listening with them. Currently I have a very old Velodyne 10" non-servo subwoofer running with these speakers. The Velodyne is way to slow to do these 2-way speakers justice. I am considering either building a sealed sub around a Morel driver or purchasing something that is ready built. What subwoofer would you recommend to pair with the Sonus's ? I'm trying to keep this as affordable as possible while doing the Sonus's justice.