recommendation for speaker wire to soften the highs if possible

I think that you have 3 good choices. 1. Delund's Western Electric style cable. 2. Western Electric cable, there are lots of fakes now, this is a good source for the real thing;

https://www.m.ebay.com/sch/i.html?sid=tajacobs&isRefine=true&_pgn=1

3. Goertz speaker cables, but you MUST get the zobels to connect to your speakers. Their almost purely capacitive so they can make many amps oscillate without the zobels, but being almost purely capacitive they have far less effect on the sound.

The problem is that all these are really neutral cables. Brightness, if we assume that your gear isn't just bright, will only get better if your cables are the cause. You can also buy or make deulund interconnects. Deulund has copied and have arguably bettered the old Western Electric cable, but if you don't have high efficiency speakers 91db or  better efficiency, you can get 10 gage W.E. cable from the above source, or use Goertz. Back to if it's not your gear, or your cables, it could be a power issue. As a technician I always believed that with a reasonable power supply that noise on AC lined shouldn't be a problem, until I had a problem myself. I spent several thousand on ICs trying to fix it, but what did fix it were isolation transformers, one hospital grade for low current devices and one for my amplifier. Going to better ICs and speaker cables will increase enjoyment anyway, so it can't hurt, though it can make issues more obvious. If you decide to go to tubes the best bang for the bucks, IME will be Roy Muttram for reasonably good sound, his kits built by him or you are very solid, but Don Sachs has taken them and using the best components really maxed out very well designed gear. Roy says he can do the same, but he just isn't anal enough about tweaking. He's a great designer, so it just depends, if a little less than the best, 80 to 85 percent of the way there, for as good as it gets, the equal of 20K or better gear for just under 6K, Don Sachs is the guy. Don is beyond generous with his insight as far as matching his gear to other gear, and he'll make you Delund cables for your gear if you are purchasing from him at a good price. Obviously I make my own, but it was Don and one of his customers that talked me into it. I have Don's DS2 preamp, and Roy's ST80 amp, but I am upgrading it, I should have just bought Don's amp, I never designed tube gear, worked on tube cameras decades ago, but I have made expensive mistakes, just buying the amp Don has designed would have saved me not just time, but money too. YMMV.

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!

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.

Nonoise, 

"I have about half a dozen speaker cables and every one of them sounds different. It’s just the nature of cables. A designer is going for what he/she thinks sounds best or can sound best for a variety of different systems."

True, that's why I like the cables engineers made in order to minimize such issues. 

Geoffkait, good point. That's another reason I like Goertz speaker cables. Square wave response is a solid indicator of a component's ability to deal with transients, quick sharp sounds, such as cymbals. Western Electric cables and Goertz speaker cables were both designed by electrical engineers. This page shows the difference between Goertz and more normal cables to a square wave.

http://www.bridgeportmagnetics.com/contents/en-us/d62_MI_AG_Speaker_Cables.html

d2girls, cable has inductance, capacitance, and resistance. These are the 3 qualities used in electrical filters, it's what speaker crossovers use to separate frequencies, so unfortunately cables are very capable of distorting sound. That's why I use Duelund exclusively for interconnects. Western Electric used to be AT&Ts sole source for cables, and when wires were necessary for telephone communications people spoke to and fully understood each other from coast to cost using Western Electric (W.E.) designed cables, and Duelund has slightly improved upon it, likely due to improvements in the purity of the metals used, but regardless, they are the most neutral cables I have found. For speakers I am using W.E. because it's less expensive, and while I haven't A/B'd them yet, I have used Goertz for decades, nothing other than W.E. has been listenable. Mind you I have crazy revealing equipment and I run a full range driver, so what comes out of my amplifier does not go through a crossover, and it's amazing how much of a difference that makes! I have reworked many crossovers using V-Caps to by-pass larger caps, and replacing crappy resistors with Mills non-inductive wire wound resistors, but regardless how good the components are, you are still filtering the sound and trying to match drivers. Nothing beats a good full range driver IMO. Frankly Don Sachs talked me into trying it, he made my preamp so I knew that he knows what is what where sound is concerned. Before I bought the preamp we spoke extensively about component quality and such because I wanted to be sure that I wasn't going to regret buying his DS2 preamp... Anyway, cables absolutely impact the sound, though if your equipment is sufficiently colored, it's possible to mask that, at least to some extent. The more neutral your gear, the more obvious the differences between colored and neutral components.

d2girls, IME, usually it's an equipment issue, but I concurr 100% about cables not being an answer. At best they are a bandaid.

I've said all along that it is likely an equipment issue. I had a similar issue with my DAC, and cleared it up with a reasonably priced hospital grade isolation transformer, that and Deulund cables, which are just good neutral cables, should be under $300 or so, and in many system would cost far less that trying new gear. YMMV