jwillans wrote: Is this configuration possible with the MJ Acoustics?
It would seem not. The only inputs to the sub that you would want to use are the unbalanced RCA (phono) sockets. The simplest way would be to drive the Behringers and the sub in parallel, but this would involve unbalancing the inputs to the Behringers. In a domestic environment I wouldn't expect this to be a problem though. What kind of connectors are you currently using from the souncard to the Behringers?
Alternatively, if you have a pair of unused outputs on the soundcard you could use these as a separate feed to the sub.
Nick