recommendation for speaker wire to soften the highs if possible

For the googlth time, systems in a room as individual as a fingerprint.

Blanket recommendations are not worth the bits to transmit them.

See http://ielogical.com/Audio/CableSnakeOil.php for examples of how the same cable reacts with just amplifier and loudspeaker XO order.

When one adds in low level interconnect, all electronic and room colorations buying a component on recommendation is odds worse than craps.

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.  

Audio Note LX interconnects are worth considering to address your issue 

@johnsonwu do you know specific size and voltage for the cap (and resistor)?

I just corrected nasty, harsh tweeter highs/fatigue. 
A friend brought over some Dueland wires and they did the trick. 
Myself and a group of serious audiophiles tried all the things suggested in this thread. 
I was to the point of breaking out the ice picks and sticking them in my ears...
Just  saying, you might seriously consider the Grannying option. It shouldn’t be too expensive and you may not need anything more. ( I don’t know Grannying, I’ve never had contact with that person). 
I’m with you. Most likely the Monster cables are the problem. 
Good luck. Keep us posted.