Recommendation for an audiophile speaker cord that is not "bright"?

Give Wilson a call and see what they recommend. They do a lot of shows every year, not sure who they partner with. Off the top of my head I would try Acoustic Zen Hologram, Kubala  Sosna and Kimber cable.

If you like the cables in general but find the brightness an issue, you might try different plugs.  The top of line IEGO 8095 is rhodium over silver and this plug has had a smoothing effect in my system.  My speakers use Eton metal dome mids/tweeters and can have more bite than I like.  My power cords are DIY solid core copper with silver plate.  I usually use Oyaide 004 plugs but have added the IEGO's to a couple cords and the detail is still there and the highs are at least as open and transparent but somehow smoother without the bite.  Something to consider.

Anticables, really tough to beat
then if ya want to spend a bit more money an Audioquest with DBS, stock to copper

have fun, enjoy the music and the search 

Cardas are known for warm sound, except the "Clear" model.

Look into WyWires. All copper... no silver or silver plated. I used to be a big fan of silver cables but as my humble system has become more refined I’ve gone back to copper. WyWires have great balance and don’t draw attention to anything; they just get out of the way. Only listened to the Sasha’s with a Gryphon Diablo 300 in a store, but it was very revealing and I would have preferred a little more warmth. Not sure what model but they were running but they were using Transparent Cables. Sound was too hot for me but was very dynamic but not fatiguing but still too hot for my taste. Source was an Aurender N100 I believe. Not knowing the rest of your system or your room it’s hard to know if it’s the Nordost or not but my experience with other Wilson speakers is they are very revealing and I feel they require more system matching than other speakers. I use a mix of Nordost as well but not the upper level stuff but I do find they blend well with WyWires. You’re just going to have to experiment and find what’s right for you. What is the rest of your cable loom and what speaker cables do you personally own? You mentioned the Nordost are a friends. Upper level Cardas might be another line to look into.