I have done a comparison today between the Null Audio Vitesse (8 wires of OCC cryo copper, with upgraded mmcx connectors and 3.5mm plug), the stock copper cable and the stock silver-plated copper cable using my iBasso DX150.
The bottom-line result, ranked by sound quality from best to worst: Null Audio Vitesse > stock silver-plated copper > stock copper
Audio material:
"I.G.Y." by Donald Fagen, from "The Nightfly" album, and - as we are in mid-december- "Merry Christmas Mr. Lawrence" by Ryuichi Sakamoto, from the "Three" album. All 44.1khz / 16 bit flac CD rips from my collection.
Some notes about each cable:
The stock copper cable is kind of "easy" on the ears, it's got a nice voicing but sounds also a bit dull, veiled and muddy. Bass has the least impact and there's a lack of overall detail. It performs best in the lower mids, piano chords hit with a good amount of weight.
The silver-plated copper cable is much closer to the Vitesse in detail retrieval and articulation. There's clearly more power in the low frequencies compared to the stock copper cable. It's slightly bright in general and sounds exciting at first, but becomes a bit fatiguing over time.
The Vitesse is clearly the best cable out of the three. It sounds like a superior cross-breed between the stock copper cable and its silver-plated sibling: it's just as comforting as the copper and as detailed and exciting as the silver-plated cable, but without the imprecisions and lack of involvement of the former or the annoying sharpness of the latter. It's imaging is bigger and more accurate than what both of the stock cables can deliver, and it brings out great overtones and articulation in all instruments. It's also more comfortable around the ears and more flexible, so it's the obvious winner in this comparison.
Generally speaking, the stock cables are not bad though. None of what I wrote above should lead you to believe that the CL2 is horrible with them on and I still think that RHA did a good job in that regard. It's just that the CL2 is so damn revealing that it can improve considerably with better cables. It is one of the few things that actually get better if you throw money at it (ha ha).