The ext phono output would go into an aux/cd/tuner line input.
Not sure if this helps:
Help with Marantz NR1510
Trying to help a friend with New AV unit. He ran his rega TT strait in to the phono stage fine. Then he wanted to compare that to the external phono stage he has. I told him to go into the audio in assignable 1 , 2 or 3 but when he tried that no sound. Does he have to activate those inputs in the "audio in" section of the NR1510?