I've added a SVS SB2000 sub to this recently and it is everything the reviews say it is.....amazing. I moved the Threshold to the top of the rack and the sub is underneath everything. Was worried about this placement but it has only added to an already great soundstage. Overall I'm really happy with this setup.
Next is a proper rack, looking hard at the Salamander as it will allow the sub to be underneath the rack on the floor...not on a shelf.