recommendation for speaker wire to soften the highs if possible

Strap a 4 ohm resistor and a 0.33uf cap wired in series across the speaker terminal/binding posts. 
That creates a very crude low pass that rolls off highs.  Let your ears be the judge.   Cost you $2 or so.  Better than buying $500 snake oil.  

@nasaman  I can start a dissertation on how to use LTSpice to run simulation on how an LC filter strapped in parallel to an LCR load (speaker) will work to roll off high frequencies at which point by how much of a slope, or I can just suggest with about $10 investment you can get some caps from parts express and a few pairs of resistors, one can try them out for a binary search to get where one wants in terms of taming the harshness of the tweeter.  The simplified guide is:  The higher the value of the cap the earlier it will roll off, the lower the value of the resistor the steeper the roll off curve is.  I know it is never as satisfying or confidence-building as spending thousands on cotton wrapped oil impregnated cables with a mystery box, but it does save some time.  https://www.electrocube.com/resistor-capacitor-networks