Sorry for not checking back sooner, hj - sorry. Life continues on much the same way: Every few months, the toilet starts hissing and I can see the water in the bowl has tiny waves. I put a little vaseline on the flapper and it is quiet as a mouse for another few months. The same thing happened with a new flapper as with the old one. It would seem to me that this means that the surface where the flapper seals must be uneven. Fortunately, and coincidentally, a Kohler rep told me that my circular overflow valve is too narrow and they now have a rectangular repalcement "flush valve kit" that is free from them. So, I am planning on replacing that and hoping that will fix the problem for a long time.
I am a little confused about the rim wash issue as someone (below) suggests that a different type of value works fine KWsundell below likes Fluidmaster 400A Fill Valve. You seem to think the 1b1x is the only appropriate repalcement if you want the rim wash. Is that true?
My conclusion is that, whenever water somehow leaks, it is possible that, if you have modified the rim wash from the original using the 1b1x tube, you will get very noisy siphoning action back and forth leading you to believe that the problem is somewhere else than where the flapper seals. Also, this 1B1X valve seems to have a propensity to spray in random directions and, given the holes Kohler puts in the back of the tank (to hang them to dry after painting, I've been told), this is an accident waiting to happen. I covered those holes on the outside with duct tape and that seems to work.
I have two more toilets just like this but they work fine, even though the fill valve sprays in random directions on those too, but with the duct tape, it is not an issue.
Comments solicited. Thanks all.