Don't put your turntable in a corner -- this is a bass node and will maximize interference with your table. Ideally as others have noted the cable to the phono stage should be as short as possible.
One option to consider is getting a small phono stage that you locate next to the table connected by a 1M cable - then run a longer cable from the phono stage to your amp. Of course your amp has a MM phono stage built in so you could go with a head amp and a low output moving coil and the run the longer cable to the Marantz. Even a step up can help -- I currently run 2M cables to my step up then another 2M to my phono stage and have no issues
One option to consider is getting a small phono stage that you locate next to the table connected by a 1M cable - then run a longer cable from the phono stage to your amp. Of course your amp has a MM phono stage built in so you could go with a head amp and a low output moving coil and the run the longer cable to the Marantz. Even a step up can help -- I currently run 2M cables to my step up then another 2M to my phono stage and have no issues