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=13. 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.