Ok- you’re not a fan of Aurender then you say you could do better for the money.
Please suggest alternative servers
What is your DAC?
The Innuous Zen recommended above by tump350 is a great alternative for a turn-key solution, as are the Antipodes products. There are also Melco, and some other more esoteric products (Sound Galleries comes to mind).
I think there is a lot of confusion when it comes to computer based audio. My suggestion is to examine the meaning of the word "server", and with respect to music there is actually no inherent difference between "servers", as a "server" simply "serves" another computing device with an index and catalog of files. In a "pure" sense a server simply acts as a storage location for the files.
A server can also host a client, which is a software program requesting file information from the server. Or, the client can be a completely separate device thousands of miles away requesting files from the server. It doesn't really matter.
Where things get complicated, and where the differences really are within digital audio (or video for that matter) is the process between the "client" (the application requesting the files) and the "server" (the application providing the files). Ideally the "server" software interfacing the files to the "client" (receiving device) is transparent, or in other words does no modification to the file before it is transferred to the client. Some servers "transcode" or modify the file data before being sent out.
An Aurender "music server" is actually both a "server", and a "client". The files reside on an internal hard disk and are indexed by an FTP-based server software running on Linux (Ubuntu 14 or something along those lines, a few years ago). When playback is requested by the "client" (music player daemon for Linux), the files on the hard disk are transferred internally using an internal SATA bus to an SSD where they are cached for playback. The Aurender app is an FTP-based control point software which allows you to select which files the Aurender queues from the HDD ("server") to transfer to the SSD. The "client" is also known as the "player" software (in Aurender's case, Linux OS and music player daemon).
The "client" or "player" is actually the real determining factor here, as it is what translates the file (Aurender cleverly uses the term "render", which is quite accurate) into usable audio playback information to be delivered downstream to possible additional digital signal processing, and ultimately to a digital-to-analog convertor.
Essentially you are already doing the same thing with JRiver using a Mac, which I suppose you didn't need me to explain. There are some nuances to what Aurender is doing, but try a dual-boot to Ubuntu on your Mac with MPD (and no other programs running) with the same files. It's a bit more work, but money saved...
In a sense Aurender is not much different than a typical computer running Linux because they use standard (non-bespoke) bus interfaces and PC componentry which is nothing particularly special. The parts Aurender uses as storage, RAM, and CPU are all off-the-shelf (and quite outdated in some cases); the exception is that they do have a specialty supplier for their own mainboards, which is commendable.
Those of us who have spent a lot of time with digital music playback will be able to elucidate why a better approach is, and has been, to focus on the "client" and not the "server".
Aurender themselves are very aware of this as I would guess a majority of their users have moved to Tidal or Qobuz streaming, meaning the "server" is what Tidal and Qobuz are providing, and their "client" (player) software is still retrieving files in a cache on the SSD. The extra dollars spent on storage are a waste at this juncture.
What audio and music enthusiasts need to realize is that audio companies packaging computer parts one can purchase from NewEgg or Amazon into their products should consider what they are offering and claiming before doing so. Better architecture already exists, and instead of learning and adapting, companies like Aurender are remaining steadfast in their flawed ideas about what they believe are flawed ideas. (Their refusal to adapt Roon in any meaningful way is evidence of this, and I am not a user nor a fan of Roon in particular). Unfortunately as network topology and implementation improve, the problems Aurender was trying to solve with their customer base will end up backfiring as now a user is stuck with a brick that sounds worse when disks are spinning (this is noticeably audible and demonstrable), thus rendering (no pun intended, surely) the product more or less useless if used in a true TCP/IP "server" manner (UPnP, DLNA, or other Peer-to-peer file transfer). Further the product was never designed to be used in a NAS-like fashion meaning the ultimate reliability is unproven as what I would actually call a "server" (i.e. always on, reliable, immediate).
Why not continue to use your Mac with an external hard disk or USB/thunderbolt storage (sky is the limit there), park it on your network (in other words keep it as a server), install a UPnP software (sounds like you already have it with JRiver), and use a UPnP renderer to deliver digital to your DAC?
Purported sound quality differences between files stored on any number of commodity NAS products or PC/Mac computers while using a true TCP/IP network-based delivery system are negligible, assuming the network, server and client software, and associated components are configured properly.
The main issue with USB is noise. The main issue with SPDIF is clock synchronization. Audio over IP is the real future, just look at the list of manufacturers implementing the Dante protocol into their product.
https://www.audinate.com/meet-dante/dante-manufacturers-list
Aurender has already apparently implemented Ravenna (similar to Dante) which makes their product seemingly even more obsolete - why buy a box with a bunch of useless storage on board if that storage can be remotely located with absolutely no "downtime" due to "spinning disk noise"?
Besides Dante and Audio over IP, look into Linn, Naim, Sim Audio Moon, Sonore Rendu, Auralic, Lumin, Bryston, DIY Raspberry Pi, BluSound based offerings, and other similar UPnP/ethernet based products. Many of them have SPDIF outputs for external DACs. Most of the built-in DACs on the products which offer them outperform the standalone DACs many people are using anyway (mainly due to elimination of the two problems mentioned above with USB and/or SPDIF).
I would probably have a more specific recommendation were I to know which DAC and the rest of the system, and network and such as well.
Anyway feel free to PM me if you want to discuss further, or I can continue here.