You could try Powerline products http://www.uk.tp-link.com/products/?categoryid=206 if you don’t find a long enough cable.
I would move the Pi onto the wired network first to see if this can improve matters. For example is the Denon Streamer Hard Wired ?
From the theory side the audio network path is completely different:-
MP3 from NAS
NAS >> RPi (over wifi) = one wireless leg
Spotify >> RpI
Spotify Server >> Router (by wifi) >> Nexus 7 >> RPi (by Wifi) = 2 wireless legs
The best solution would be if there was a Spotify Service on the NAS Drive. I don’t think they have arrived yet.
In theory the UPnP QOS Quality of Service Service on Routers could reserve bandwith for comms Router (by wifi) >> Nexus 7 >> RPi (by Wifi). Never tried it so cannot speak from the done it camp. 
As for the difference between Volumio and gmediarenderer = not my area, but definitely seems an important variable too 