Looking for a speaker cable with a relaxed sound.

To further @millercarbon's point, and although I suggested a cable that brings some qualities to the table, it's been my experience that you shouldn't try to fix sonic problems with cables.  Cables should enhance the qualities you already like about the components.  Any time I've tried to fix sonics with cables it was always just a long and costly exercise that inevitably precluded selling the offending component (speaker/amp/etc).

Wireworld Eclipse 8 would fit that bill.

I would second this suggestion.  I have heard the Wire World "Eclipse" level XLR interconnects.  They have very excellent resolution, but they are just very slightly laid back (definitely not forward or bright).  I think the same would go for the "Eclipse" level speaker cable, which is pure copper.

I would NOT go to the "Silver Eclipse" level or above, which contains silver / silver clad conductors.

Purist Audio Design is another good recommendation for this.

Not answering your question, but I assume you've played with toe in and tilt to see if you can achieve a better balance?

This is the problem with speakers that aren't custom tuned to your ears.

Just kidding.  :)

Depending on your price range (and contradicting the posts above),
Wireworld Platinum Eclipse 7 speaker cables are the most relaxed, least bright cables I've ever heard and they're all silver. 
That model is at least 50% off of original prices, if not greater.

So you’ve all this detail and spent a bunch extracting it. Now you want to dull it back down a little? Try this experiment. Romex 14/2. The white one from Lowes/Depot. Use only the black and white wires.