JBG, if it's correct that the 4802 inputs in question give the same result as if connecting a separate amplifier, then the setting of the volume control on the 4802 is irrelevant. The volume control operates at the preamp stage and this is bypassed if the connection is directly to the amplifier stage.
However, these external multi-channel analog inputs are typically meant to feed feed voltage from a player, etc., into the preamp stage. If this is the case here, a preamp output would be fed into a second preamp stage, possibly overloading the 4802 stage and causing distortion. At least, the volume control on the 4802 would have to be turned down substantially to reduce the voltage input from the MRX.
