I think you're going to have to learn a little about servers and networking in order to work out what you want to do and how to achieve it! First, a few definitions...
- External disk: disk drive(s) in an enclosure that attaches to a device (e.g. a PC, TV, set top box...) via USB, that the device can use as though it were its own internal disk.
- File Server (aka NAS): disk drives(s) in an enclosure that connects to your network router/switch via an ethernet cable so that any other device on the network can use as though it were is own internal disk.
- Media Server: indexes and catalogues your media files, which can be stored on an internal disk, and external disk or a NAS, and makes them available in an organised menu structure for other devices to use.
- Media Player: connects to a Media Server and allows you to browse your collection and play any item on the device you are using.
- Media Controller: connects to a Media Server and allows you to browse your collection and sends instructions to another device to play an item.
- Media Renderer: receives instructions from other devices to play an item.
That's a simplistic overview but I think it works! Any device can take on one or more of these roles. Let's have a look at the devices you have got and explain what roles they can play.
- DS213: At it's most basic, this is a File Server. It also has a built-in package called Media Server which, if you enable, will create default folders for you to store your media in and it will index them. It also has a web application called Audio Station which is a Media Player and Controller. You can use any web browser to browse your collection and either play it in the browser you are using or send it to a DLNA device on the network or a USB device connected directly to its USB socket. There is also an iOS & Android app called DS Audio so you can use a phone or a tablet to browse your collection and either play on that mobile device or send to any other device that the DS213 can see.
- JRiver: This can act as Server, Player, Controller and Renderer (in any combination!).
- Preamp: This is effectively only a Renderer.
I assume that what you want to do as to have a browsing mechanism for your media files that are currently stored on your PC and play them on the preamp. If that's the case then there are several ways of doing this.
1. Leave the files where they are on the PC, import them into JRiver's library and use JRiver to browse them and send them to the preamp which will be connected directly to the PC via USB. In this case the DS213 is redundant.
2. Move the files to the DS213 File Server, import them into JRiver's library and use JRiver to browse them and send them to the preamp which will be connected directly to the PC via USB.
3 Move the files to the DS213 File Server, enable Media Server, and use JRiver to connect to the DS213 to browse your files and send them to the preamp which will be connected directly to the PC via USB.
4. Move the files to the DS213 File Server, enable Media Server, and use Audio Station and/or DS Audio to browse your files and send them to the preamp which will be connected directly to the DS213 via USB. (It will show up on the DS213 as "USB Speakers" I believe). In this case, JRiver is redundant.
Advantages/disadvantages of these approaches?
If you choose JRiver as the Media Server (1 & 2), you are using probably the best media server there is. It is more flexible and powerful than any other, you will be able to catalogue your collection as you wish (as opposed to the DS213 which can only index by album, artist and genre) and you will have more options in expanding the way you listen to music around the house, including video. As a company, JRiver is less "faceless" than Synology and you will get better help and support and faster bug fixes here. However, it does need a Windows PC to be on and running all the time and the DS213 as a Media Server (3 & 4) is far less power hungry and quieter than a PC.
With 2, 3 and 4, you will have no backup of your media files in case of a "disaster", unless you already have other options in place. With option 1, you could use the DS213 as a File Server to take regular backups of your PC.
Everyone here will have different approaches. For what it's worth, I have JRiver on a PC upstairs in the "computer room" and my media files are on a Synology DS209. I use JRiver to play media locally on speakers attached to that PC, and I use the iPad JRemote app downstairs and elsewhere in the house to control JRiver and send media to other devices, such as my TV for video or my hi-fi amp (via a WDTV Live Renderer) for music. In other words, this is option 2.