My experience is that cable upgrades can improve the sound but it’s not a significant improvement.
But upgrading the source (DAP) can bring significant improvements.
Case in point: I have cable upgrades for 2 IEMS (not 64 Audio) and my Focal headphones, all with 4.4mm balanced connectors.
Yes there was an improvement. It wasn’t huge and IMO maybe not enough to justify the cost of the cable upgrades.
But I was also going from single ended to balanced on my DAP. Was the improvement from the cables or the balanced connection? I don’t know.
However, I later upgraded my DAP to a Chord Hugo2/2Go (Not exactly a DAP but close enough?)
The Hugo2 only has single ended outputs so I had to revert to the stock cables.
The improvement in all aspects of sound quality was massive. Significantly more than I’d experienced with cable upgrades. Thus I feel no desire to upgrade the cable for my u18s.
I appreciate there are many variables here and it’s hardly a ”scientific” comparison.
But my experience leads me to think your money may be better spent on a source upgrade before a cable upgrade.
Edit: In thinking about my post I realized I compared an upgrade of several hundred dollars (cable) to an upgrade of several thousand dollars (DAP).
So let me change my opinion to:
$1000+ investment - Source/DAP
<$1000 investment - Cable or save your money for a source upgrade