when you say turntable straight into the mpc are you saying you're connecting the rca outputs of the turntable directly into the mpc with rca->1/4" adaptors?
The adaptors are no problem but you'll need to route the turntable through some sort of phono preamp, dj mixers are the most common.
If none of these are issues for you I'd check the levels coming in to the mpc, bring the gains up on your source first (pushed up to about 3/4 or a little higher will be loud without distortion) then raise the rec gain knob on the mpc til you have a strong signal (bumping around the max bar but not clipping into it)