Sonus Faber

Need, advise on integrating a "SUB" into the equation. Great speakers, just got them, But a little shy on the bass... What if any, SUB ('s) should be considered? have the room for a stereo set up, if necessary? Have been thinking about the New Thiel "Smart Sub", or REL "Stentor III's" or...But, this is the first time around with the Sonus Fabers (two ways) The Cremona floor standers I had didn't require the extra punch. Any advise or experience will be of great help before I spend the bucks. Thanks to all for the effort.
Craig