By default, XMPlay uses the AAC decoder that comes with Windows. So if/when xHE-AAC support is added to Windows, it will also be supported by XMPlay. There is also the XMP-AAC plugin, which is based on the FAAD2 library. I'm not sure if FAAD2 is still being worked on, but if an update with xHE-AAC support does become available at some point then the XMP-AAC plugin can be updated to use it.