I was noticing some shimmy in my 1-month-old XB-600. While stopped, I applied the front brake fully and rocked the bike forward and back. There was a distinct "clunk," indicating looseness in the steering tube bearings. (I also checked the triple clamp bolts, but they were adequately tight.)
Access to the bearing nuts is restricted, but I was able to tighten them up by tapping them gently on a land with a metal punch, with the lock nut tightened the same way, only with a bit more force.
Steering is much more positive now.