What cables can lower my forward treble.

@knorpi doing some RTA anaysis will save you the needless cable spamming. I'm hazarding a guess thatthe data will show clesrly areas of harshness

Everything in the chain makes a difference, and cables can make or break a system's desired sound qualities.  For instance, in my experience, silver cables, especially interconnects, can tip a system's high frequency response to the point of it being a little too hot, bright and fatiguing.  You have nice components that shouldn't produce negative high frequency effects.  I would first try swopping out some cables (power cables/interconnects) and see what happens.  I think that should do the trick.  Happy listening.          

I’d like to know if you are using RCA or balanced connection between the Cary and the Plinius. Swapping one for the other might change the characteristics you’re trying to obtain by using a different brand of cable which I don’t think is going to change your systems tonal balance very much. It’s possible the interaction between the Plinius and the Cary is where the problem lies. Driving a solid-state amp with a tube preamp. Impedance mismatch.

Also, Dali usually recommends that you point your speakers down the length of the room rather than towed in toward the listening position. You might experiment with that if you haven't already.

Switch to an all copper cable (speaker & interconnects) no silver and use the balanced inputs and outputs. 

I got the last bit of high end harshness and fatigue out of my system with super tweeters and some other tweaks.

The super tweeters were a loan from a friend but I never gave then back, mine are Golden Sound JSMR from Japan which are no longer made. There are others on the market which I couldn’t vouch for, but the thing I didn’t expect is that any harshness from my speakers was almost totally gone. I think (theory) is that the excess energy/ultra high frequency sound that we can’t hear is still present in the signal going through your speakers tweeter but isn’t reproduced and causes distortion, but the super tweeter is able to output the sound. 

The other thing that really helped and made things more smooth and analog sounding was Synergistic Research fuses. I was super skeptical, then I bought one for my amp and couldn’t believe what a difference they made, now they are in every component that has a fuse, they make things sound smoother and more analog and real.

The last tweek that also helped was buying a Schuman wave generator, there are tons on Amazon for like $10, but the ones I have are the Nobsound brand. They were $50 on Amazon and have a bamboo wood housing so they look nice. They don’t seem to sell them anymore but the brand is now Douk audio, no more wood case but similar product. Those really do make your room sound better by just plugging one in to the wall and setting it in your listening room, supposedly the best location is 5 feet above the floor in the front, I have one in front and one behind the seating position and they make everything smoother, and more analog. I even did some blind testing with my non audiophile cousin and she could even hear the difference turning it on and off.

It’s a really cheap tweak unless you buy the Acoustic Revive Japanese one that’s like $700 which may be better, I have some acoustic revive stuff and it’s really good.