That is pretty much what I figured. What is really annoying is that whenever I do anything with the Tivo remote after I alter the volume with the receiver remote, the volume keeps going up up up up.... A workaround in case anyone on the forums searches in the future for this unit is to hit a non-Aux button (stop, play, etc) after changing the volume (both steps referencing the Nexxtech receiver remote). This seems to prevent volume change issues when using the Tivo remote.
I'll keep subscribed to this thread in case anyone chimes in with a working programming code.