Like a lot of upright bass players I have a David Gage Realist pickup on my bass and play the majority of my upright gigs through a Markbass head and 210 cab. I’m not familiar with the J-tone pickup but I’ve rarely had feedback issues with the Realist. I also regularly play the same bass through a big Ashdown rig with a 6x10 cab with no problems, plus it’s been through a bunch of festival hire rigs (usually Ampeg SVTs and big 8x10/4x10 cabs), again no problem.
It always helps to have a bit of distance between the cab and the bass to avoid feedback issues, more so as the volume levels go up. The only times I’ve had an issue have been when I’m right on top of the amp as you mentioned, or when playing a hire bass with a crappy pickup.