gonk, I ordered the MIC SPLITTER/COMBINER from partsExpress along with a 15' and two 3' XRL cables. I just can't isolate my ground hum and when I run just one sub with the Velo XLR LFE output, it's silent. As soon as I use the unbalanced outs on the SMS, I get the ground loop again.

The MIC SPLITTER/COMBINER looks like the best option for my situation, giving me all balanced connections from the pre to the SMS, to the MIC SPLITTER/COMBINER and to the subs. I like the fact that the MIC SPLITTER/COMBINER also offers a ground lift switch on the box itself. I'm confident this will solve my problem.