recommendation for speaker wire to soften the highs if possible

@erik_squires  +1
Room first, then electronics, then cables.

@gunmetalzin - what version of Monster? They made many @ multiple price points.

Where are the speakers placed in what shaped and how furnished room? At what height relative to ears. Try sitting lower or higher. Forward / back

Try angling the speakers and / or tilting them back. Try reversing L & R to have tweets inboard or outboard.

You may want to try replacing the HF bi-wire link with a 5Ω 5W resistor and see if it helps.

Ignore most everything else here.

I suggest thinking of your audio system as a whole. Something is causing harshness. There are two ways to treat that.

First, as Elizabeth suggested, attenuate the offending HF with ferrite beads. Cheap, easy, and probably effective, unless your speakers are the offending component.

Second, find the source of the harshness and fix it, usually by replacing something. Ultimately that will give better sound, and so I do it - but that's because the bug bites me daily. Warning: it's time consuming and costly, because it could be source, amplification, speaker, room.

To test the speaker cable hypothesis, do as noromance suggested.

ieales has some good suggestions too. Lots of soft things in the room, like carpets, can help.

I agree that Dueland cable might tame the harshness you are hearing.  I don't see a preamp in your system, and if you are on a tight budget, I  would heartily advocate hearing a Schiit Freya preamp with the tubes upgraded to Shaguan 6SN7 tubes and you will get taste of audio splendor.  The next and final step would be a Don Sachs preamp. :-) Consider room treatments as well.  

Get Granny to make a Dueland set and jumpers. You will not regret it!