I haven't had a Triaxis for a couple of years, but I believe that if you use a device in the FX loop, it has to have a stereo return if you want to feed a stereo
power amp. Which always struck me as odd, since the Triaxis has a mono send, right?
So I guess the options are to split the mono send into L/R before going into your FX box (assuming your FX box is stereo) or split the output of your FX box into L/R before going back into the Triaxis FX returns.
--B