Out of those 3, I would buy the KK. It has 3/16" "I" beam sides that goes all the way back to the discharge shoot, VS the Howse rolled under sides. I see this as a tremendous strength advantage. They both have similar smallish gearboxes. I can't comment on the Tennessee brand.
I have an older Howse that has a skidplate welded underneath down each side. Where that plate isn't, the sides are bent and torn. I see that happening to the rolled under side, that took the place of the welded side. That being said, I've mowed 3" saplings of oak, cedar and mesquite with mine with no problems, other than the torn up sides. The gearbox oil seal went out quite quickly but I filled the box with grease and 300 hours later has no problems.
I'm looking for a 6'er too. I think I'm going to go with the Rhino SE. It is sturdier built and has a much bigger gearbox for 1250.00 here in my area. That is 350.00 more than the KK I saw. It also has a front deflector which I feel is very important. The KK has no where to mount one and would need to be modified to install one. Not much of a big deal, but extra work.
Just my .02, MP