What cables can lower my forward treble.

Toeing the Speakers outwards towards being 180 degrees aligned to the listener is a great tamer of a overbearing projection of high frequency.

More importantly a revisit to the geometry used for the Speaker in the listening space does not cost money, unless you call in the services of a removal company.

The Pro's that are to be found from such a activity can far outweigh any other methodologies being considered.  

Also don't overlook the age of the Speaker and a High Frequency that shares Two Drivers, the Crossover can be showing its lost its youth and is not blending the Done/Ribbon as per the optimised design.

http://static.hifi-forum.de/produkte/336/bedienungsanleitung_dali-euphonia-ms4.pdf

https://www.youtube.com/watch?v=k1qvQ-ffuYE

There is a service referred to on the Gon, where a individual will design a upgrade Xover for a Speaker. 

+ 1 to @soix . I had the same problem with slightly forward treble and slightly bright. I run a tube pre into a tube amp and rotate a SS amp occasionally. I was able to IN MY SYSTEM AND ROOM tube roll and eliminate silver in my interconnects. The final touch was playing with speaker  toe in. That being said my issues were not that severe and I already had a pretty dampened room that lacked acoustic treatments. As others have said acoustic treatments matter as do other changes in a more extreme case. I’m a mid Fi guy , the correct signal tubes and for me Morrow interconnects are great , but that was with my conditions. My personal opinion is that cables alone MIGHT not be enough , but YMMV . I purchased a pair of Schumann generators and a pair of linear power supply’s off Amazon for $100 and use them on my headboard in my bedroom. One of these days I’ll put them up behind my stereo and see if I can hear a difference. Happy hunting , Mike B. 

+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.