i just got this unit and connected it to my polk audio rti6's. The radio sounds pretty good, not too warm, not too bright... just right. When I connect the cheap Sony turntable i have the sound sucks. It plays ok as long as you don't stress mid range like heavy rock... i mainly want to listen to CD's and LP's. My iron maiden records don't sound too good. Is it the receiver or can it be the turntable? are there any good turn tables one can buy nowadays?
The Sony turntable isn't very good and the phono preamp in the Marantz isn't very good. That equals not very good sound. Your sound would improve with a better turntable with or without a phono preamp. A phono preamp on its own won't improve your sound by much. I would look into the ProJect Debut turntable with a built in phono preamp. Good luck.