@stuartk my understanding is you have tried different cables between dac and amp and that didn’t solve the issue. Those were DH Labs cables.
In my experience DH Labs cables are brutally honest and lean slightly on a brighter side of neutral.
I will give you an example from my most recent cable swap/upgrade experience…
I was using Acoustic Zen Absolute Copper XLR between the Bricasti M3 DAC and XP22 pre, as well as between the XP22 and X260.8 monos.
The AZ Absolute Copper were a bit too warn for my system with the latest upgrades. So I called The Cable Co and tried the following: Audience AU24SX, Analysis Plus Silver Apex, Kimber KS1126. I had on hand AZ Silver Ref II and the Nordost Tyr 1 and Tyr 2 that my friend loaned me.
There was a significant increase in sibilance with the AP Silver Apex. A bit less with Audience AU24SX but still present, there with AZ Silver Reg but no sibilance with Kimber and Nordost Tyr 2. I never experienced sibilance in this system with the AZ Absolute Coppers. There was always a super smooth midrange with delicate treble but the bass was too plump with my Wilsons and the Pass monos. I struck gold with Nordost Tyr 2 XLRs. My speaker cables are Audience AU24SX
So the moral of the story is - you still have not done due diligence with the cables.