Oh yeah, I should have thought of the USB input! I guess that I'm not worried about the touch screen, though, as it seems more intuitive to use a tablet or smartphone running the BluOS app to control the unit, select music, etc....I can't see why anyone would prefer to walk over to the preamp and use the touch screen. But that's just me!
For those unfamiliar with the C658, it does have a USB input -- sort of: it accepts a flash drive and can play music files from that. But that's not what most people have in mind with a USB input.
But if you are wondering how you might connect a music library, note that you *can* connect a USB device, e.g., a hard drive or similar, to most routers and then stream the music from there to the C658. In fact, the BluOS software will index the music on such a drive. That's what I've done; I find this method particularly handy because I can add music to the library from my desktop computer (because it, too, can see the router), so I never need to mess with the hard drive itself. Those who think that you need a fancy music server with gobs of memory and a super processor to improve the sound will be horrified with this solution....
But as to the original question (what could be added), AlexanderC has nailed it.