I would be highly surprised if iTunes could sync with non-Apple devices like a Astell & Kern device. Chances are you'd have to sync the Astell & Kern device outside of iTunes.
Also another thing to consider here is the files in your iTunes library itself. For example, if your files in iTunes are m4p files (obtained either from the iTunes Store or from Apple Music), they're DRM-protected which will only play in iTunes, iPods/iPads/iPhones, etc. You wouldn't be able to play those on a Astell & Kern device. If they're MP3s, M4A (which is either AAC or Apple Lossless) or any formats like that, as long as they're not DRM-protected and the Astell & Kern supports it you may be able to get it working... of course, like I said you'd probably have to do the sync outside of iTunes as I don't think Apple would support third-party devices like that.
JRiver Media Center itself doesn't run on any iOS devices like iPads, however you use it in tandem with JRemote, which will run on an iPad like Jim mentioned, to stream music from a PC or Mac running JRiver Media Center to an iPad running JRemote.
