Hi !
I forget to mention, that many times there where a complain in that zone, I vefind excessive pressure on the glide bolt (tone is too percussive and close, not enough legato), or at reversE not enough (bland tone,too open, mating problems exacerbated)
It is often difficult to trace on those actions with aluminium rails and light key frame (it will be evident on a Steinway)
RX frames warp high in summer so it may be the second solution more probably.
the fact that you where able to mufle the tone with some kind of moderator does not mean that the voicing is responsive, unless all the piano is too percussive and thin.
Regulation and checkIng are modified with uneven glide bolt pressure, but tone radiation also, and particularly, what you feel under your fingers, which is 50% of the pianist hearing.
be sure to take that in account in your evaluation.
I would press on the balance rail and see if the front of the keys are moving, before any other search.
aftertouch may be minimal if you want the high bass to be pleasing, Rx often have a large dip, if the keyfRame warps high, the firstbregion where it is noticed is there.
better lower the extremes if possible, than lower the center glide bolts and add paper punchings under the keys. But any solution apply well as long the key height is not too far from the original (65mm ?) measurement.
how much key dip ? above 10,25 you are in trouble unless you like a somehow mushy keyboard.
May be Kawai changed their punchings now but they where very soft so you can have end of letoff in the punching, as on one of the traditional German action setup. they dont like too firm punchings either, as the attack have to becut down on Kawai