Use the Tap Tempo & your ears man, or go into the sample edit window and shorten the length if it's too long. MPC can match any sample if it's cut right and your skilled enough with swing, timing, & time signatures...
There's also a lot of variables in PT to think about. And I think that's more so where the problem lies.
Such as when you say "make it a loop in PT" what process are you using...Are you just setting loop points and have the transport set to loop, or are you trimming the actual sample accordingly to where it naturally loops.....
Also, is the tempo in PT set to the tempo of the sample, or are you just leaving the tempo in PT at 120bpm?
As well, how are you exporting the sample from PT? Dragging & dropping? or Bouncing/Rendering or Saving a copy of the recorded sample. I know that in Logic if you "save a copy" of the file it saves the entire thing, but if you "save selection as" it saves only the chosen portion of the sample.
All of these can make a huge difference to your workflow. Seems like it would be easier for you to just sample into the MPC and make it loop there.
...cheers...
