Maybe it's lack of appropriate talent, but I know that in the studio, the only way I can get close to an acoustic sound is for the engineer to take my track, convert to midi, and run through a high quality acoustic plug in. Live, I have many basses and string combinations to choose from and I know I can't get a sound that would be close to an acoustic. Again, maybe limitations on my part.
I can get the sound close, but not quite there. Like mentioned above, it won't be the same to an upright. But, i can get it close with a crappy p and some nice broken flats, and a few amp adjustments. When I'm going for that upright sound i do pluck differently. Of course, not the same, but i can get it to sound close.