If merely this arrangement

nothing > 755 > speaker cables > speakers

gives you a mild RF reception and demodulation situation, and to help determine if the 755 is the main culprit even without cables, can you disconnect all cables from the 755 except for one speaker cable that is very, very short (perhaps less than a foot, if you can manage it) connected to a loudspeaker temporarily sitting right behind the amp and listen for the station? Does the station come through all channels if you test the amp one channel at a time with the short speaker cable? Just thought I'd ask.