What cables can lower my forward treble.

OP  What cables can lower my forward treble.

Your high freq maybe emphasized due to laid back mid-freq. Make sure what you need is changing a cable. As a reference, below sounds well balanced.  MAGNEPAN 2.7i / HEGEL Viking

Below is Wavetouch speaker cable and mid is forward.

Wiim amp ultra (upgraded)

Alex/Wavetouch audio

@12many You are the right. Adding capacitance reduces high frequencies. It is the basis of passive crossover designs.

@squared80 : I agree! Wire as a tone control is wishful thinking. 

@jasonbourne71 

"Adding capacitance reduces high frequencies. It is the basis of passive crossover designs."

Capacitors in a crossover network "pass" high frequencies and attenuate bass. In essence to prevent bass frequencies from overwhelming a midrange or tweeter's voice coil and burning it out.

There are simple first order or 6db per octave types that may only use a capacitor but probably include a resistor to trim the tweeter's output and bring it in line with the woofer's so the speaker system doesn't sound overly bright.

Then you have second order or 12dB per octave which are the first LCR or inductive, capacitive and resistive where the inductor trims the upper range of the bass, the capacitor filters out bass to the midrange and tweeter as in the case of the first order and again you have resistors to trim the output of the tweeter and midrange if one is included so that the two driver's output doesn't overwhelm the output of the woofer creating a shouty and bright balance.

Third order which is another LCR type or 18dB per octave often referred to as Butterworth design.

Forth, again LCR at 24dB per octave Linkwitz-Riley design. 

The designer of the crossover may even combine multiple techniques listed above to achieve a particular design goal for the speaker he's engineering. There may be other techniques as well used for crossover design that I have not addressed.

These cables? "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" https://www.synergisticresearch.com/cables/foundation-xl/

I have silver interconnect cables here in the cabinet right now.

• If remove my Cardas Clear Reflection copper interconnects [with Grade 1, 99.9999% pure oxygen-free copper (OFC) conductors] between my Cary tube preamp and my SS amp -or- my mono tube amps, [and install similar silver cables], I can effectively create the added "hi frequency energy" you refer to in your original post.
• Not all, but some silver interconnect cables can take away the body and warmth and add the upper hi freq energy you refer to. 

Can you borrow on loan some quality all-copper interconnect cables to try?