its a can of worms indeed. However, when you have optimized the source, amp, headphones etc it does make sense to revisit it. With the level of components in the chain we're talking about here (GS-X mk2, Kitsune Holo L3 etc), it makes it easier to hear the differences in cables as well.
I made my own XLR pair with 22AWG occ litz with a carbon infused core wire that I was able to source in the past (unfortunately, not anymore). It sounds excellent and the one I currently use. I also have the 24awg neotech Silver/Gold wire (not litz) on the way along with Abbatron xlr plugs. Have heard great things about this wire, so going to try it out as well.
I've also made headphone cables with the Jena 22awg cryo wire and found it to be excellent as well.
With USB, the geometry and shielding is a lot more important than the actual conductor itself IMO, so I picked the Oyaide which seems to separate power data lines and similar in geometry to the iFi. It also had the best noise floor characteristics out of the half dozen cables I tried, including much more expensive ones.
With I2S, the blue jeans belden is the best I've been able to find for the money. Great build quality and sounds better than the wireworld hdmi which sells for like 10x more.