The UPnP setting I was talking about is on the router Admin page, implying that the setting is to enable or disable the UPnP method for administering the router.
Admittedly, the documentation for the Linksys WRT1900ac is a little confusing:
HOW TO CONFIGURE UPnP
UPnP (Universal Plug and Play) allows devices connected to a network to discover each other and automatically create working configurations. Examples of UPnP capable devices include web cameras, online gaming applications, and VoIP devices. UPnP is enabled by default.
To configure UPnP, do the following:
• Log in to Linksys Smart Wi-Fi.
• Under Router Settings, click Connectivity, then click the Administration tab.
• To use UPnP, select Enabled (default) next to UPnP.
• To allow changing router settings while using UPnP, select Allow Users to Configure.
• To prevent local network users from disabling your Internet connection through UPnP, deselect the Allow users to disable Internet access checkbox.
• Click OK.
I have disabled this setting without affecting AirPlay and my PS3 is still listing the media server built into my NAS (hanging off a switch on the network).
The router also allows one to connect a USB hard drive to it and serve media from it. Those settings are in the "External Storage" page of the settings. Here is the language for that in the manual:
HOW TO SET UP YOUR ROUTER AS A MEDIA SERVER
A media server lets you share media content across your network. Your router can act as a media server if it has a USB drive attached and if you have UPnP AV (Audio and Video)-enabled or DLNA (Digital Living Network Alliance)-certified devices in your home. Examples of UPnP AV-enabled devices include digital media players, gaming consoles with a built-in media player, and digital picture frames.
For example, if you have a digital media adapter that sends content to your entertainment system, and if your router’s set up as a media server, then the digital media adapter can access your router’s attached USB drive.