Question: "Will AirPlay stream up to 96/24 or 192/24 or will it downsample everything to 44/16 even if the receiving device is caple of higher resolution playback?"
Answer: Airplay will stream the 24/96 content to receiving devices like the Marantz NA7004, but only from your computer, not from one of the portable Apple devices. Those portable devices, like the iPhone and iPad are internally limited to 16/44. You can use your iPhone or iPad to control the content streaming from the computer to the receiver.
Also, when you listen to music on an iPhone or iPod, the music is not at the 16/44 resolution, it is lower. Apple provides a DAC in each device that does not deliver the full 16/44 resolution. 16/44 is equivalent to CD quality and the Apple Lossless format. If you want that quality from your iPhone, iPod or iPad then you need an external digital dock that decrypts the USB output and converts it at 16/44. Some of the docks available can process up to 24/192, but the iPhone, etc. are still going to restrict you to 16/44 maximum.
You can also use a streaming system over wifi from you iTunes library directly to your iPhone or iPad. Try Zumocast for that, but again, limited to 16/44 and you would need the digital dock as well to get that resolution.