Confirmed - cat7 network cables didn't improve playback on my setup...
NAS ->cat7 -> router ->cat7 -> iMac + Audirvana -> USB -> DAC
Your result is not surprising. Me changing Ethernet is equivalent to you changing USB.
Next - upgrade the router :-)
Why if Audirvana caches the track before play? You should not hear any improvements changing anything before your iMac.
Now if the DAC caches, one should not hear any differences changing anything before it. Not even a belt driven transport. If you build them, they will come. LOL!!!