streaming from JRiver/NAS FLAC files, Amazon Music HD, and Spotify.
You could use a Raspberry Pi 4B and free open source software such as Moode, or Volumio to do all of the above except Amazon HD, and you can replace Amazon HD with either Qobuz or TIDAL, but at a higher monthly cost.
In that scenario the computer you currently use becomes a server, still running your instance of JRiver with the FLAC files on the NAS, but it streams them over your LAN to the Raspberry Pi acting as the "player", or what's also variously called an endpoint, streamer, or renderer. The Raspberry Pi 4B then connects to the Topping D10s DAC via USB.
That Raspberry Pi 4B, in a nice metal heat sink enclosure, comes in south of $100, and lets you keep using the Carver amp/preamp you already have, as well as move the actual server computer to a different location not near the stereo if you so desire.
If the DIY aspect of the above is unattractive, you can get a finished product that will do the same thing in the $399
iFi ZEN Stream, which is in my opinion a better overall value than a NODE at $549, if you already own a DAC that you like.
Amazon HD is closed source and likely not going to be made available to anything but the big hardware partners that are willing to shell out for a pricey license, or of course Amazon's own Echo devices.