What cables can lower my forward treble.

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

The  best answer is often redicing the reflection off of your wall surfaces, but if it must be cables try mid range wire world

+1 @eminence_audio 

I make my own cables in that exact way for years with excellent results. 

@knorpi Is there anything you would like to add?