There is also the need to properly load the phono cartridge of your turntable for flat response. The Moving Magnet (MM) cartridge may need 47K with 250pF termination. Does the audio preamp have the ability to provide this load? Also, you need to assure you have the proper gain for optimum signal transfer, as MarkT alluded to.
If you are interested in playing 78RPM records, then the RIAA isn't accurate as this format came after the 78RPM era, and its needle profile.