Just to clarify, do you have the Marantz set up as a preamp, with your Cd player plugged into one of the Marantz inputs? This would give you the CD player and the FM tuner as sources.
From there, do you have the Marantz Preouts connected to the MA252 inputs, using it as the amplifier in this setup?
If the CD plays fine through both speakers, but the FM doesn't, that tells me the issue is in the Marantz receiver somewhere.

