My current and previous Kioti came with a steering wheel tilt mechanism. I would check to make sure that it is fully locked in, just to eliminate one possible variable. The second thing I would check, and I am not saying you haven't, are the nuts and bolts underneath the tractor. Vibrations make these pieces of equipment shed nuts and bolts like a cat shedding hair. While you are at it, check the torque on your lug nuts on your tires, just to eliminate another variable. The book will say to do this periodically.
My last tractor had this bizarre harmonic distortion that was related to the tire pressure with a filled tire. When driving down the road it would vibrate like crazy, 5 more psi changed it completely. You may want to ask about the tire pressure on the filled tires (if they are filled). I wouldn't rule anything out.
Just throwing some ideas out there for you.