I currently use an HTPC directly connected via HDMI to my AVR. However, I also have networking capability so I checked into how this is done with my receiver. A few things to do:
1) Your AVRs should have an source input called "network", "media server" or something like this. You'll need to select it as the "source"
2) If you can view the AVR in your network, see if you can go to the "device home page" - this is how I was able to navigate to my music collection on one of my PCs - in other words, use a PC on your network to identify your AVR/s through network management
3) Ensure you have set Windows to allow media streaming for the AVR
4) Also, make sure you have the latest firmware update for your AVRs
In all, streaming via the AVR interface is painful (after using a dedicated HTPC for so long) but workable. Let me know if any of these helped.