If you can, and feel it's worthwhile - Rotate your wheels/tires from front to back (put both fronts on back and backs on front). Then see if anything feels different - it might even get worse if the culprit is a badly balanced back when it's on the front. I have noticed X350's are very unforgiving of out of balance wheel/tire, but its subtle and seems to be better/worse depending on road surface; I had one new set of Michelin that absolutely refused to be balanced; Michelin admitted it and exchanged them for free.