What cables can lower my forward treble.

Try Veritas. It does not add warmth, but it makes high-end notes smooth and liquid without compromising detail. I have Optimus (I²S) and Magnus (XLR).

I also tried the Cardas Clear digital (USB) cable; while it smoothed the high-frequency notes, it compromised dynamics and softened the low-end response.

I never expected cables could alter sound characteristics to this extent, but I am now convinced.

Just curious if you only started having this issue after adding the Hegel CD player?

I have the Dali Euphonia MS5's and I agree with knotscott, as I added felt around the border of the tweeter module and it does help with that tweeter high frequency edginess. And yes Dali's perform best with 0 degree toe in. 

@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.