I don't know about your Marantz, but your typical AV receiver tuner has a 3 gang front end, an IF section consisting of two 180 khz filters, and an AM/FM chip that contains all the functions that used to be performed by three separate chips.  The detector is simpler and is not as low in distortion as older tuners.  It is no wonder  that you might prefer a better designed tuner with lower distortion and better separation.