Wired vs. Wireless

WiFi might produce electrical ambient noise, that you most likely already have if you use wireless router, but wired connection can inject electrical noise directly into the system.  Also WiFi transmits only data and as such it cannot have lower or higher dynamics or amount of noise.  Important is what happens to this data later.  Data is most likely compressed, so it has to be decompressed and timing has to be attached.  In case of my Airport Express it comes out as 16 bit S/Pdif stream at 2x44.1kHz word rate.  DAC converter  can adjust/follow it for D/A conversion or just ignore it, rescan data and output it at new unrelated clock (upsample), as my Benchmark DAC3 does.

hickamore, It's the wire, conducting electrical noise from computer or router/network to the DAC.  Even optical isolation in Ethernet network has few picofarads across - enough to inject some high frequency noise.  I would limit number of necessary wires (power wires for USB are not necessary if DAC is self powered) and use shielded cables to reduce pickup of ambient electrical noise.  I would also avoid placing network wires next to power wires. 

I'm not saying one method of delivery is better than another.  I use wireless, because I couldn't route Ethernet cable across the room.  I had a lot dropouts, until I switched to 5GHz band.