If you are using ASIO and sending the usb out and monitoring through the MOTU I see no problems there. If you're using the Control Rig for everything..for both input and output then that's even better. WDM is generally not as good compared to ASIO for this type of application.
I wonder if there have been similar complaints on the unit from others. You might be getting a little more headroom if the MOTU is set to 24 bit and the Control Rig is set to 16bit. Maybe there are different ways to set up the Control rig as pertains to effects...does the control Rig default to some kind of factory settings that are different than the settings you have used? Good luck in finding the problems...maybe someone else will jump in with more suggestions.
Anyone else here using Control Rig?