NuVo does need all eight conductors.

I did a couple of systems using Xantech stepping remotely controlled VCs. They are clunky and big, basically multi-relay switches, but work like a charm. They have volume on, volume off, volume up, volume down, and commands direct to each volume level. This can go to one VC or as many as you like -- it's a matter of routing.

The amount of wires depends on what you use to get different IR commands to the central switching location. Three wires would get you IR sensing. Xantech Keypads would work, too, as, I think, would Niles.