I heard its like impossible,and rarely done well,because once a song is mixed properly it like its apart of the beat.So you'll prolly end up taking away quality,instruments,and/or distorting them.
yeah if you wanna sample basslines then just eq out everything 'cept the lows but if you wanna keep some of the bassline's "personality" mess around w/ the mids until you get some of the mids of the bass.