What cables can lower my forward treble.

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

I have built custom cables for the past 20 years and tested everything to learn as much as possible. I won't suggest a cable brand, that is up to you; I'll just outline the exact properties you need to research. These will get you as close as possible to your goal. 

1. Avoid silver. Use only copper.
2. Don't go larger than 20 AWG for interconnects and 13AWG for speaker and power cables to keep skin effect to a minimum. Skin effect adds extra treble energy.
3. Single conductor and not multistranded. Non-linear inter-strand electrical conduction in multi-stranded can introduce high-frequency issues and grit.
4. Soft temper of the copper conductor is the best. This lower hardness, excess energy and ringing in the sound.
5. Most importantly, look for a cotton dielectric on the conductor; it has a triboelectric of 0 (dead neutral), and this will prevent any electrons from being added to the signal through a static charge. + or - triboelectric charges both add electrons to the conductor by the end of the cable run. You can immediately hear the results of this.

Jason