So, from your post, it looks like you were able to make the RX-1800 a "master" of sorts through your connections. I'm assuming you used a mixture of Pre-outs and input sources to accomplish that. Is there an option to simply have the RX-v671 act as the master by switching your connections around so that it's pre-outs are driving the other two receivers? If its network ready, does it support DLNA or any other media server type communication? If you're able to make it the master and stream from your PC to the networked receiver, you may be set.
Alternatively, you may have to find a wifi streaming box with RCA or other outputs that would serve as your media server interface and plug that into your RX-1800.
I'm just jealous that you were able to get approval for 3 receivers. I lost a leg over my single refurb Denon.