I have the L-509x and have run a Hana SH (HOMC) thru the MM input and now run an Audio Technica AT33SA (LOMC) thru the MC input.
Both sound fantastic! John Atkinson's measurements in the Stereophile review of the L-509x get good marks.
The only reason I'd consider an external phono would be for flexibility in loading options. But, pick the right cartridge and just spin the LPs and enjoy the music!