I've been using a dedicated network attached storage, aka NAS, device for about 8 years now. I originally wanted one in order to easily share files between several different computers on my home network. Prior to that I had just used an external hard drive attached to my wireless router, but that wasn't a very robust or speedy solution for my needs. Once I got the NAS all set up, I discovered a myriad of uses which I hadn't even thought of before. Now I can't imagine living without it.
At the time I went with a Synology unit, only to discover it was kind of a lemon. All sorts of connectivity problems and after just a few months it died completely, forcing me to make a switch. I was ready to go with QNAP until a friend pointed me towards Asustor; I wasn't familiar with the line but have always enjoyed Asus products in general, so I gave it a go. Many years of faithful service later, the unit is still going strong, but I found myself needing more storage than the 2-bay design could offer.
After evaluating various current models, I again found Asustor to offer the most bang for buck. I chose their AS6404T 4-bay unit and am very pleased that I did. This thing is a great NAS and also offers some unique audio-related features which make it relevant for music enjoyment - to my knowledge, no other brand can do what Asustor does in this area.
As a NAS, the AS6404T is very competitive. $549 gets you a 4-bay device with superb build quality, a relatively potent (for a NAS) Celeron J3455 quad-core CPU, and a full 8GB RAM as standard. In that price range one typically finds far less RAM and/or an ARM-based processor which isn't anywhere near as powerful as the J3455. You generally want your CPU above a certain threshold in order to comfortably handle more advanced features like video transcoding (or music playback with a large library). While Synology and QNAP have finally launched a few reasonably competitive devices, there is still usually a compromise involved in CPU or RAM... not to mention the audio-related connectivity which pushed me towards Asustor.
(I'm using stock photos here, my 6404T lives in a messy media cabinet right now and nobody wants to see that)
The 6404T is very full featured: front panel LCD, dual Gigabit Ethernet with Link Aggregation, a pair of rear panel USB 3.0 ports plus another up front, a USB 3.0 Type-C port, an HDMI 2.0 output, and a Toslink jack as well. This means it can do all the usual NAS duties but can also work its way into an audio system like no other NAS I've encountered.
I set mine up with 3x 8TB Western Digital Red drives in a RAID5 configuration, plus a 250GB Samsung 860 EVO SSD for caching purposes. I don't know how necessary the cache drive really is but with SSD prices so low these days, I just went for it. My choice of RAID5 gives me 16TB worth of storage plus a decent level of redundancy - I have everything backed up externally as well so I'm not worried about it. I know some of the newer devices from Synology and possibly QNAP give 4-bays plus an M.2 option for the SSD cache... that would be nice, but it wasn't a big enough factor to sway me.
Asustor calls their operating system ADM for Asustor Data Master. I find it competitive with Synology and superior to QNAP, though obviously they all have their own ups and downs... and they are all constantly evolving from month to month. ADM is simple to use and has quite a few useful apps available (well over 200 of them by my count), and is vastly more developed than you'll find using a budget device like Buffalo or Zyxel. In my experience it is generally up there with Synology's DSM system which many consider the industry leader. Again, as a regular NAS, I find the Asustor very competitive overall.
I'd like to shift the focus to music playback as that's why this review is relevant to HeadFi in the first place. Store your music on the NAS, connect it to your audio system via Toslink or USB or HDMI, and use one of the several available playback apps to control your music. It works surprisingly well in my experience. Depending on the app, you'll either use a phone/tablet or else a computer (via web interface) to interact with things, and the experience is surprisingly full featured. And the sound quality? Excellent.
Asustor NAS devices support external USB DACs. Since ADM is based on Linux, most DACs work well, but I'm sure there are exceptions. I have yet to personally find a DAC that doesn't play any music at all, though certain models have issues with DSD. As with other Linux-based devices, sometimes the thing requires a reboot before it plays well with a newly connected DAC. A friend tells me his old NuForce uDAC (maybe uDAC 2 or 3?) won't even do CD quality playback but I can't confirm that. In the case of a USB connectivity issue - or just an older DAC without a USB input - the Toslink connection comes in handy. It does hi-res PCM up to 192kHz assuming the connected DAC can handle that (some Toslink receivers top out at 96kHz). Or, if you wanted to use an A/V receiver or one of the few DACs with HDMI inputs (the NAD M51 comes to mind), the HDMI output is an option as well. Between the three different outputs, there should be something for everyone.
On the software side, again we encounter multiple options. The Hi-Res Player app is the first one I tried; it has a nice interface, handles standard CD quality FLAC as well as high resolution PCM (24-bit/384kHz) and DSD playback (via DoP with a USB DAC). It can be controlled via web interface or theoretically by any MPD control app such as DroidMPD (Android) or Soundirok (iOS). I say "theoretically " because I have yet to figure out the configuration settings needed to get those apps talking to Hi-Res Player. Then again I haven't really tried very hard as I quite enjoy the web interface using my Surface Pro. The only issue I find is a limit on library size - I have over 5TB worth of lossless music on the NAS and Hi-Res Player seems to give up rather than properly scanning it all. If I copy a portion of my library to another folder and point the app there, it works just fine, but I don't know where exactly the limit might be. In my experience a lot of apps have particular issues like not working with certain special characters (the & sign for example) or not handling sub folders which are nested too deeply. I haven't fully explored this to figure out what Hi-Res Play dislikes or if it truly is just a size limitation.
Next up is AiMusic, a first party app from Asustor. It has a simple yet fairly attractive interface which allows for intuitive operation - 2 minutes in and I felt like I had been using it for years. Again I had issues with the app showing my entire library but I can't imagine the developers had 100,000+ lossless tracks in mind when they built it. When I switch to a smaller library, it all works well, including proper album art. This app also allows for easily streaming the library to my phone, giving me access to my music from anywhere in the world.
The final player to mention is SoundsGood, which I'm happy to report does support my entire library. I use the web interface to control it and am quite pleased with the results. I later learned that AiMusic actually uses SoundsGood as the playback engine so I have no idea why the web interface shows my whole library yet AiMusic doesn't. Documentation on this (and most other apps) is fairly sparse - it took me a while to figure out how to change output settings from HDMI to Toslink to USB, as the actual button in the app looks a bit different than what they show on the Asustor web page... and it requires a double-click to open, which is not documented anywhere.
Browsing through a massive library is just a tad slower than I'd like, but still very reasonable overall. Using a more reasonable library feels totally smooth and responsive. For someone not accustomed to a superior UI such as Roon or a well-configured JRiver, this is a perfectly fine way to handle all music playback. Even as a rabid Roon fanatic I still find SoundsGood to be perfectly satisfactory in the room where my NAS lives - which is handy since I don't have a Roon endpoint in there at the moment. I have the NAS connected to an iFi nano iDSD Black Label which typically drives custom IEMs directly... it's a great little system that sounds very nice indeed. The NAS itself is on the quiet side but if I was using open headphones the faint hard-drive activity noise might become a distraction when playing certain music. With closed headphones or IEMs (or speakers), or with the NAS in some kind of cabinet/enclosure, it's not an issue.
Speaking of sound, I still can't get over how impressive this thing sounds as a transport. Whether running USB or optical, I can pair it with an extremely high-end DAC without thinking twice. Despite not being a "dedicated audiophile device", I really don't hear any drawbacks. It's better than all the laptops I have in my house, and better than my Surface Pro as well, with excellent imaging and fine microdetail. This is likely due to the Linux-based OS having very low overhead, but still... there's no fancy power supply or shielding here, and with 3 hard drives spinning away it doesn't really come close to best practices for a transport. Regardless, I really like what I hear. Is it perfect? No, I like the SOtM sMS-200 better for overall tonal richness and dynamic heft, though it's surprisingly close overall. That's not bad company to be in.
I got a little crazy and added an iFi iSilencer3.0 between Asustor and USB DAC. With some pairings I noticed a worthwhile improvement, with imaging becoming even more focused and treble sounding very slightly more natural. In other cases there wasn't any change at all. It really depends on the DAC being used. Best case scenario was that I'd call it just a hair behind the SOtM sMS-200, which again is what I consider a very high performance transport. I was all set to try out the iFi DCiPurifier2 to hopefully close the distance when I realized the Asustor draws 7.5 A while the iFi only supports 3.5 A - I decided against it. Still, for a general purpose computing device, the sound produced is outstanding - I used it to feed a Wyred 4 Sound 10th Anniversary DAC ($4,500) paired with a Cayin HA-300 tube amp ($4,000) with no complaints at all.
I do have to mention the main drawback compared to Synology or QNAP. That being the user community, which for those two brands is thriving but for Asustor is sparse at best. If you need configuration help or have some questions/concerns, you are typically better off dealing with Asustor support directly. The community just isn't there in this case. Asustor support is responsive but I still envy the community-driven nature of the Synology ecosystem. That means Asustor is probably more appropriate for users who have at least some knowledge of computers and networking, who can figure out the majority of things on their own.
One example of this is Roon. A friend runs Roon Core on a Synology DS918+ which sports the same CPU and RAM as my 6404T (but his cost more and he had to add another stick of RAM to hit 8GB). Roon feels quite snappy on his setup, despite using on a relatively slow older Crucial SSD for the database - probably due to the overall library size being relatively small (he relies on Tidal augmented by about 100GB worth of hi-res favorites). He tells me the install was a breeze. I can probably get Roon Core up and running on my setup but it would require working with Docker or Virtualbox and I'd have to figure it out myself. I probably won't bother as my huge library seems like too much for the hardware, but it would be nice to have the option available in a simple process like Synology does.
Despite that, I feel that I still haven't scratched the surface of what the 6404T can do. I intend to mess with Plex now that it has Tidal integration, to see how well it runs in this context. There's Nuclear Music Player which appears to handle web streaming from Soundcloud and many other sources. Then there's Spotify, Amazon Prime Music, Twonky, BubbleUPnP, and various other DLNA/UPnP options to mess with. And that's just the music-oriented stuff - it already runs Emby Server for movie streaming, syncs with Google Drive and OneDrive, and a plethora of other mundane functions that I'm forgetting right now.
Yes, most of this could be handled equally well by another NAS (probably at a higher price though). But the audio capabilities are what really make Asustor special in my opinion. If I'm going to run a NAS anyway, why not make it double as an unexpectedly nice transport? Because, as odd as it may seem, that's what the 6404T sounds like.
EDIT - March 2019
Roon has arrived! Thanks to enthusiast/developer Christopher Rieke, Asustor now has an app for Roon Server. Using a 256GB Crucial SSD for the Roon database, the AS6404T performs surprisingly well. Even with my nearly 10,000 track library, it feels as responsive as any NUC or standard PC I've used for the job.
I can upsample to 384kHz PCM as well as DSD128 without issue. DSD256 is a bit of a stretch (sometimes it's fine, sometimes things stutter), and I haven't done any multi-room or EQ tweaks which can take a lot of CPU horsepower. But for what I'll call "standard" use for a single listener, the Asustor is definitely adequate.
This pretty much seals the deal for me - the AS6404T now does everything I could possibly hope for, and I'm thrilled that I chose it.