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Since adding weight should increase the sag, I would think you want to back off the preload, not increase it in an effort to duplicate the effect of the chain weight. Basically, measure the sag with the extra weight, then back off the preload without the weight until you get to the same sag measurement.Once your sag is set, maybe try backing off your damping in small increments. Ride it after each adjustment, taking notes after the ride and see what you like.
You're very light. I'm 150lbs and had to back off on all my dampening and rear preload considerably. I have no idea how much the PO weighed, but unless he was literally racing this thing at 170# it was too stiff.
Too much spring for your weight is a real possibility