I've had the Marantz HD-DAC1 on my desk since 2014. It is a lovely little device, provided you know what it is intended to be, and to do...and what it is not intended to be, and to not do. This isn't meant as a be-all end-all reference HP amp, nor is it even a be-all end-all DAC. And it is an order of magnitude cheaper than an equivalent function set of reference separates would cost. What you get is Marantz engineering that nicely pairs its various components to produce a relatively transparent and neutral sound.
Like the name implies, it is primarily a DAC, the HP amp section is secondary but well-enough done. Don't expect much in terms of analog inputs, this is made for digital input. Nobody expects a reference DAC to have balanced XLR analog inputs, and this is no exception. That isn't what it is made to do.
Computer-driven, as intended, the HD-DAC1 is good enough to show a good deal of sensitivity and transparency to the source. Imaging is good, you can locate instruments, hear fingers slipping on strings, etc.. Lower quality MP3, MPEG, AAC, etc., are annoying to listen to, the HD-DAC1 reveals all their flaws, especially in the highs. Somehow Audirvana Plus has a great synergy with the HD-DAC1 and really makes this unit shine, with CD quality AIFF files I get a very pleasant listening experience and I don't really yearn for something better.
I also do voice recording with a Focusrite Clarett audio interface and mix with Adobe Audition, and playback with HD-DAC1 as a monitoring setup easily reveals all the acoustic flaws of my room and unintended sounds of my mouth. Marantz says the HD-DAC1 works optimally at 192 kHz, and unlike my music library I often work at 192 kHz with my own recordings. I also have a portable recorder that produces up to 192 kHz WAV files with low noise floor (it also records native DSD). With the portable recorder WAV files played through the optical input of the HD-DAC1 sound great, DSD is sublime, but I can't really tell whether this has more to do with the recorder vs the HD-DAC1 since I don't have any DSD music files to compare with my AIFF files as a reference for comparison.
As far as headphone pairings, it has plenty of power to drive anything in my experience, and the pot is clean. The HD-DAC1 lets the cans speak for themselves. Warm cans sound warm. Neutral cans sound neutral. Bright cans sound bright. And so on. I like this behavior, and in this sense at least, the "Marantz flavor" doesn't impose itself so strongly where the rubber meets the road.
The annoying thing is that Marantz makes 2 other "HD-" boxes that look very similar to this one, but they have a lot of redundancies, and it isn't clear that any of them are meant to pair together as a system. There is a Marantz HD-CD1, a CD player (but little/no SACD or DSD functionality as far as I can tell) with its own DAC section...so not much need to pair with the DAC section in the HD-DAC1. And then there is a HD-AMP1, a modest integrated that powers loudspeakers, has a headphone amp and...also has the same DAC, so no need to pair it with the HD-DAC1, either. As far as I can tell, the DACs in all of these are the same ESS Sabre chips. So I have to say that I really don't get what they were trying to do with this line...if I were the boss of Marantz, I would have a HD-CD1 that has full DSD and SACD functionality with the reference DAC, digital inputs, etc., and output to balanced XLR...there would be a HD-PRE1 that was a pre-amp (maybe with a headphone section) with balanced ins/outs, and then the HD-AMP1 would be just a couple of modest mono blocks in a box. Heck, one could add a HD-REC1 turntable to the ecosystem along with a balanced output to a phono section in the HD-PRE1. Anyways...I digress.
The HD-DAC1 is good stuff. One day I might send the fixed out to an integrated amp and loudspeakers, but I haven't done that yet. In that role, it could serve as a decent DAC...but since everything is all wrapped up in a nice tidy self-contained package to sit on my desk as a HP DAC-AMP, I'm happy to keep it there. I've noticed that used prices on HD-DAC1 have fallen quite a lot...if you can get one for $500 USD or less, I think it is a very good deal.