Playing files from a NAS share.
Ok.
Just so you know it, the feature that you are using has nothing to do with UPnP standard protocols nor with DLNA.
The feature that you are using is called SMB (Server Message Block -- sometimes also called Samba).
UPnP / DLNA are a whole integrated suite of command and control services for content browsing, device control, and media serving, and the transactions are implemented at a "higher level" on top of the TCP communication stack, and based on the generic HTTP client / server model.
On the other hand SMB is a very specific service for directory browsing and file fetching, and the transactions are implemented at a "deeper level" on top of the TCP communication stack, and based on a specifically functionally optimized set of transaction mechanisms.
The Oppo will certainly have a different input pipeline for UPnP and for SMB (i.e. it will have different buffers, different buffer sizes, and different buffer filling and emptying algorithms), and the SMB code is probably more optimized and tightly integrated into the Oppo's OS kernel, whereas the UPnP code is part of a library supplied by Mediatek which is bolted "on top" of the OS.
So I am sorry to say but the fact that the Oppo can pull and play a 24/96 5.1 WAV via SMB, is no guarantee that it could pull and play the exact same file via UPnP pull, nor indeed that in could play the same file via UPnP push.
Notwithstanding this bad news, if you are lucky, then it could be that the Oppo's UPnP input pipeline performance is sufficiently close to the border line of performance needed to play a 24/96 5.1 WAV that if you did make a few tweaks to your LAN performance, it might be sufficient to tip the balance to where you need it.
Personally I would suggest to try if it works with a hot wired Ethernet connection (single Ethernet cable) joining your PC and your Oppo. Since in this case you will not have a router providing DHCP, you will need to manually set up both the PC and the Oppo to have Static IP addresses on the same subnet (e.g. 192.168.1.1 and 192.168.1.2 ). If the hot wired connection does work, then you can start adding back the rest of your home LAN, whereas if the hot wired connection does
not work then at least you know that the case is hopeless..