What cables can lower my forward treble.

What tubes are you using in the Cary?  That might be the cheapest/easiest way to achieve what you’re looking for.  For cables I’d suggest trying Snake River Audio as they have a refined sound that may take the edge off more than the Synergistics, and they offer a 30-day trial period so if they don’t work out you’re only out a little for shipping them back so very low risk to try.  Hope this helps, and best of luck. 

https://snakeriveraudio.com/index.php?main_page=index

You will likely find that speaker placement and setup will have much more sonic influence over different cables.  As well as placement and toe in, don't forget to try different rake angles.  If the speakers are raked back (leaned back)  even a couple of degrees that can diffuse the treble a great deal.  

@knorpi 

FWIW, I’ve used the Syn. Foundation XL power cords and a Syn. Atmosphere Euphoria digital coax cable with my Hegel H390's onboard AKM-based DAC  and did not find that combination to be bright. 

Try some Cardas Golden Cross ICs.  If that doesn't get you there, add some Golden Cross SCs.  Cardas hasn't made them for years, but there is a steady trade in this model on ebay and the audio sale sites.  They are always sought after so the good part is that if they don't work out for you, you can resell with minimal to no loss.  See also the thread on this model here--there was a recent post from another convert to the GC club.

While different cables do have specific sonic characteristics, you shouldn’t be fixing your sound with cables.