recommendation for speaker wire to soften the highs if possible

Although your problem could be related to speakers, room, or source equipment (I don't think it's the Monster cables), my recommendation to your original question is Acoustic Zen Satori speaker cables, and used Cardas Golden Cross interconnect. The AZ Matrix Reference II interconnect could also be a good option.

Look, I have spent a ton of money on cables, so let me explain something. If you find a boutique brand of cable that sounds better than Western Electric/Delund, or Goertz (Goertz for speakers only), you are masking other issues. Goertz, if you don't use their zobels, can introduce problems, but if your running a monster amp, it's a go to. Basically those are neutral, and if you shoot for neutral throught your system, you'll immediately spot problems. If OTOH, Review X says product W is it, but it's really colored, but offset issues with the reviewer's (fill in the blank), your going to be trying to cure an issue which could be multiple issues. Since product W fixed the reviewer's issue, it may exacerbate a problem with your system and add problems of it's own. That's why, though it has taken decades to actually get to where I am content with my system, I finally gave up trying for synergy and went instead for neutrality in everything. It requires a sound basis, and then using the most neutral components, a lot of Teflon caps, lots of really good resistors, etc. Fortunately it doesn't mean thousand dollars worth of ICs and speaker cables! Save yourself a LOT of money, and a LOT of headaches, and do as many here have said, Western Electric or Delund's slightly improved version if you are not running lots of current, and Goertz 11 gage copper cable if you want to go the high power route. If it's not the cables, and you are sure it's not your equipment, try isolation transformers, my DAC was ear bleeding bright until I got a hospital grade isolation transformer. If that doesn't do it, call Don Sachs, he's more up on newer gear than am I, he can advise you as to what is the probable source, or you could just call him in the first place and perhaps save both time and money in the long run, not to mention frustration!

@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

I reccomend Cardas,they do a great job of warming up a system and subtly rolling of highs. I dont recommend them for tube equipment though.

I certainly agree about Cardas, I tried that for my DAC before switching over to the isolation transformer. It was an improvement, but it was a bandaid not a fix.