What cables can lower my forward treble.

+1 @dill I have found silver in speaker cables leads to increased brightness.  Some will say this is not true if implemented well in expensive cables, but for me it is all copper.  I tried many  (including SR) from the Cable Co and chose Audience AU 24SX.  I thought they were great and got a whole loom, finding them used. Then my dealer swapped in a pair of Kubala Sosna Elation Speaker Cables and the mids and bass opened up significantly. The highs did not go away at all...just became more full range, and thus took the edge of the highs.  Anyone who says speaker cables don’t affect the systems tuneful balance are wrong IMHO.  Sure, look at components and room affects, but might as well get pleasing full spectrum cables that don’t add glare.

from SR about the Foundation XL Wires:

For each cable type, we’ve doubled the 6N pure silver Air Strings to portray music with more immediacy and resolution, improving upon dynamics and transient response in a completely non-fatiguing way.

hmmm.

You’re trying to put a band-aid on a gushing artery.    From skimming a few reviews, it’s probably your speakers. 

You could get some Blue Jeans cables, which will suck the life out of the sound.  You might be able to tame them with tube amplification, but that’s a more expensive band-aid. 

Why don’t you try listening to some other speakers and see if you can find some that are more to your liking?  For some, your speakers might be "revealing" and the Goldilocks speakers for them, but it sounds like they are not right for you. You can keep trying to live with them and spend a bunch of time and money trying to get them to sound the way you want, but why put yourself through that?

Re-positioning and playing with toe-in is a no-cost effort, but if that doesn’t work, at least consider the possibility that the speakers are the problem.

Everything in the chain makes a difference, and cables can make or break a system's desired sound qualities. 

This is only true if there are no major issues – such as speakers or room.

People on this forum love gear and cables and suggesting stuff. That's all cool. 

But if you don't do triage and understand if there is a *major* problem causing the brightness, you're going to spend money, get frustrated, and not fix the problem. 

It's just that simple.

There are multiple potential culprits for forward, bright and fatiguing sound. Some of these culprits can be eliminated at zero cost. Example - speaker placement: spread the speakers too far, place them too close to side or front walls. 
The room acoustics are critical. If you have a lot reflections you will experience brightness. If you have a bass null and don’t dial in the lower end, you will have brightness. 
OP - I’ve asked you in your other post to add a picture of your system. It will help folks here help you eliminate your issue. 

What have you done so far with the Cary?

https://forum.audiogon.com/discussions/cary-audio-slp-5-preamp-tube-replacement

DeKay