I kind of know how to achieve thump, growl, mwah, slap and so on. But, what I'm looking for is purrr. Listened to sound clips for different FX, strings, different style of bass configurations but can't find it. Some examples would be songs on Ray Charles last album. Genius Loves Company. I have "Do I Ever Cross Your mind" with Bonnie Raitt playing right now and that bass is purring. Kind of think it may be achieved with studio effects.