An easy way to test with wifi disabled is to have music player control or remote app on laptop, get usb to ethernet adapter, now have hard wired laptop. Vast majority of Android devices won't work with ethernet as most Android OS don't support ethernet.
I had also heard many cat 7 ethernet cables manufactured with ground connected at only one end, assumed this for all cat 7. Seems this not true. In any case the purpose of minimizing noise at every single link in streaming chain remains critical. Any noise, whether internally generated or picked up by ethernet cable needs to be minimized, to think we can eliminate it entirely is a fools errand. Extending coax vs long ethernet takes care of the noise issue at that point in chain, very important step.
In regard to comparing ethernet vs wifi vs optical. Noise equation comes down to wifi and optical total galvanic isolation, obviously no noise transferred component to component, ethernet may or may not transfer via grounds, and then rfi entering via connectors. So ethernet may lose a bit here, but think about self generated noise, wifi certainly high noise, you've lost some resolution at very first link in chain. Sure, wifi signal may be strong enough for no drop outs, but noise is loss of resolution no matter where generated. Plus likely there may be advantages via ISP, ethernet service may be inherently lower noise vs wifi. Ethernet vs optical advantages may be more difficult to discern, optical devices may or may not generate more self noise than ethernet, obviously optical cables less noisy than ethernet. Still far more ethernet equipment available, switches, filters, clocking to maximize ethernet jitter, noise performance. The only optical upgrades I"m aware of is the Sonore equipment, specifically the OpticalModule with it's superior power supply, filtering, clocking, which can then further be upgraded with the Finisar and/or Cisco optical transceivers. I'd like to see a fully optimized ethernet network faceoff against the optical. I'm not far off here, just need to acquire two more OpticalModules and Finisar transceivers.
@yyzsantabarbara USB vs SPDIF superiority is totally conditional on both streamer and dac optimization of these ports. Theoretically, best rendering scheme would be optimized port on streamer going to optimized port on dac. Very few streamers and dacs provide optimized usb, assume it most difficult rendering scheme to optimize since it has greatest inherent liabilities. Not difficult to determine optimized rendering ports in streamers and dacs, manufacturer will state something to the effect of dedicated power supply, filtering and clocking for the rendering port, in dacs this will generally be XMOS or Amanero usb boards, in streamers proprietary boards. Lacking optimized ports in many streamers and dacs finding preferred port is crap shoot. Generally, not as difficult to find optimized ports in dacs, our 005 usb ports being one example, finding optimized usb rendering in streamers far more rare, usually one of the SPDIF ports or possibly I2S will be best port to use. Whatever the case, certainly may be worthwhile to experiment with all ports.