The SB-850 is a companion to the KF-850. It is a tour-grade device designed for hard reliable use sonically and mechanically. They can take knocks, drops and loader-abuse much better than either of the subs you've listed. Things like dollyboards, covers and truck-pack are part of the design, but are an afterthought to the other products.
Yes, technology has improved the sonic capabilities of the under $1200 sub a long ways, so comparing a used 10-year old 850 to a new JBL is probably a losing situation, but I still think the older sub will outlast them as a working revenue-generating product.
Just my observations.