Not to necro a thread, but I just wanted to pop back in to thank you guys for the perspective. I filmed myself playing both basses and I noticed differences in posture caused mostly (I feel) by the small differences in instrument size. Very slight, but after a couple of weeks of correction, and experimenting, I've mostly eliminated the pain. There is still a little something there but it only pops up after a few hours of playing, and it's more of a soreness than sharp pain.
