Not long ago, when most home music servers used MacBook Pros or Mac minis, third-party applications like Amarra, Audirvana Plus, and Pure Music (or JRiver Media Player for Windows) helped to bring out the best in your collection of music downloads and, in some cases, offered integration of streaming services like Spotify or TIDAL. They all worked a little bit differently generally relying on proprietary or semi-proprietary music-processing algorithms, generally but not always running on top of iTunes, and frequently bypassing macOS's Core Audio functions, which had its own set of sonic limitations.
Now that many people use NAS's with bridge devices or streaming DAC's, or all-in-one music servers like the Aurender N10, you don't hear so much about Amarra, Audirvana Plus, and Pure Music anymore. That doesn't mean they don't exist or that they can make a music server based on a multipurpose computer sound better, but bridge devices, streaming DAC's, and everything made by Aurender actually are stripped down computers with an operating system already optimized for playing music. So, adding an application on top of that to further process the sound or bypass the OS is inherently antithetical to the minimalist design of such devices and, yes, even inferior to the excellent music-processing algorithms built into distributed music-serving software like Roon.
To further explain, most multipurpose computers running operating systems like macOS, Windows, and Ubuntu (a full version of Linux with a GUI) are designed to almost anything within the limits of the hardware. Thus means that run lots of processes or operations at the same time, many hidden in the so-called "background", and others right in front of you including any application you can get that will run on the computer and lots of "services" that look for input from the keyboard and mouse, manage the fixed drives, control the user interface, etc. There's nothing wrong with this. It makes the computer very flexible. However, the more different things the computer has to do at one time, the more the odds of mistakes occur. These mistakes can be very conspicuous, like having an application "crash" or just stop running abruptly, to dropping bits and pieces of data or having problems with latency (how quickly and consistently data moves from place to place). In a sense, applications like Amarra, Audirvana Plus, and Pure Music hot-wire the computer, bypassing certain parts of the OS to create better sound.
If you only need to do one or two things, like process and serve music files, you can strip down the OS, removing many unnecessary processes, increasing the accuracy and speed of the one or two things that the "music server" is intended to do. This generally results in better sound.
Devices like the Aurender N10 are purpose-built computers, without any explicit user interface, designed to process your music files in so-called "bit perfect" fashion. They connect to your home network so that you can copy music files to them, control them with the Aurender Conductor App (written for an iPad), and even stream music from services like TIDAL. Plus, an N10, as one example, can also act as bridge device by connecting itself to an NAS and serving music files to your DAC from that as well using a USB, AES/EBU, or S/PDIF connection. Aurender takes a very minimalistic philosophy and only uses DSP in the simple sense of the term, basically to move musical data from place to place with one exception: you can tell an Aurender device to convert DSD files to PCM files on the fly to support pre-DSD "legacy" DACs. However, they perform no "sweetening" of the music and can't downsample, say, a 24/192 PCM file to 24/96 to further support even older "legacy" DACs.
If you opt to use a distributed system with an NAS or some kind of data storage for your music, Roon Server running on one type of computer or another, and at least one Roon "endpoint", either a bridge or a streaming DAC that's "Roon ready", you can downsample music files to an arbitrary limit, like 48kHz, and you install plugins from third parties that can perform what I once described as "special" DSP, running more complex algorithms to improve sound particularly for cases where the recording itself or the file format is flawed. Although I don't know for sure, Amarra, Audirvana Plus, and/or Pure Musicmay have "special" DSP functions in them to enhance your music rather than delivering it in bit-perfect fashion. If that is the case, and you like that sound, any of those companies could convert their own "special" DSP functions to Roon plugins, allowing Roon to manage the metaphorical plumbing of getting data from point A to point B and/or offering control, like music selection, through any number of Apps or applications written by Roon.
More to follow ...