Could be a bunch of things. Maybe the record is old, maybe your turntables need a cleaning, maybe your needles are old and dirty, better connection to the MPC. Also, keep in mind the 2000xl aint the best sampler around. It is the best sequencer, no doubt, but its sampling capabilities leave a little to be desired. That said, I get great sounding samples from my vinyl.
Also, in addendum, yes the mixer is important if you have a crap mixer with horrible old components with bad connections and stuff inside then you are going to get crap out. I use a Stanton SF-2L LE as a mixer and it sounds perfect. If you have a newer mixer, that shouldnt be giving you a problem.