Which turntables do you own that have built-in phono pre-amps? Are the built-in phono pre-amps defeatable via a switch? Or do you have separate RCA outputs for internal pre-amp and external pre-amp? Lastly, it is unlikely that any turntable has a built-in phono pre-amp that is switchable for both MM and MC cartridges. I'm not familiar with a TT that has this feature. All of the TTs I'm familiar with only support MM cartridges via the internal phono pre-amp, if they even have an internal phono pre-amp at all.
Good Luck. Hope you figure it out and get to enjoy your vinyl.