95% of those recordings I have play(ed) on were/are done with an OLD P Bass (mine or the producer's "fave") with flat wound strings. Why? I think this - its probably safe to say that the vast majority of popular music recordings made since the '50's, especially during the Golden Era '63 - '77, featured a P Bass with flats or rounds. Obvious exceptions that were huge sellers: Beatles, Led Zep, etc.,... but, maaaaaybe, most listeners, Artists other Musicians, and most importantly MUSIC FANS and MUSIC LOVERS are, in my opinion, consciously and unconsciously most comfortable with the sound and "sonic feel" of a P Bass. It is, for better or for worse, the most "familiar" AND - Recording and Mastering Engineers love them (possibly due to that afore-mentioned "familiarity", especially with flatwounds!!! As adroitly mentioned in a previous post, maybe alot of outboard gear was designed to match P Basses... I had Lindy Fralin rewind one of his Vintage P Bass pickups twice for a "Frankenstein" P Bass I put together from eBay parts with Matty Baratta - reowund twice (stepped the output DOWN) to "perfectly" match one of Linda Perry's old LA2A's...ridiculous tweakery, but hey...it was all in the name of THE QUEST FOR TONE!!!
Peace and love - all the best in 2019!!!