Depends on what you consider "relatively cheap."

I have several software programs that allow me to accomplish this but don't know if this is what you are looking for. However, if you did go my...

I have a simple midi controller (had a CME 8) and a yamaha es rack tone generator. I simply go from midi out on the keyboard to midi in on the sound module. Then take a 1/4" cable from the output...

go to "devices" in cubase then select the output tab and then make sure that the firebox is selected as both left and right outputs.

Never mind I see you have done that. Lemme think on it abit.

They way I would hook up is this:

Midi out from Fantom to Midi in on the Rack then midi thru to the Mbox. I would also hook up to 1/4" cables to the main left out on both the Fantom and the Motif...