Here are my first impressions of this device, compared to the m903 I have been using with much joy for several years.
I use the m903 and the m9xx as a DAC only, because I have an external electrostatic amp (Stax SRM-717) for my Stax SR-007BL (i.e. a Mk.1). Thus, I cannot give impressions of their dynamic headphone amp sections.
For people unfamiliar with Stax, I can summarize the quality of my system as follows: While not top-of-the-line in the electrostatic world, mainly because the amp is holding it back a bit, my system is certainly highly resolving and high-end, and since acquiring it I have been cured of upgraditis. Yes, it's possible.
I have the m9xx hooked up to the unbalanced analog input of the m903, and have volume matched (by ear, at level 87 on the m9xx) it to the DAC section of the m903. This way I can easily switch between both devices in one second while listening by selecting either the USB or unbalanced input on the m903, and by switching the output device in foobar.
Let me first say that both devices sound remarkably similar! The m9xx does certainly not sound worse anywhere, which is remarkable for a unit with a power supply this small. I am impressed. I have always been fond of the sound quality of the m903 as a DAC, so I am very happy with the sound quality of the m9xx as well. I experienced in the past, with my Naim CD5X, what an upgraded power supply can do, hence my appreciation for the sound quality of the m9xx with a small switching power supply.
The only improvement of the m9xx over the m903 I can hear, is a slightly better spaciousness. Spatial information is presented a wee bit cleaner and better defined.
However, if I were to participate in a double blind test, I would not be able to discern between both devices with certainty. The difference is that tiny. This is not to say that someone else with golden ears wouldn't be able to do this, but over the last years I have become somewhat less interested and/or receptive in/of these extremely subtle differences between devices, as long as the sound quality is at an acceptable level (and the bar is set high). Both devices (m9xx and m903) are clearly above this acceptable level as a DAC. They both outperform any source I owned before (see my profile if you care).
Obvious improvements of the m9xx are its DSD capability and the fact the crossfeed is applied to the line outputs as well. This is a logical consequence of the fact that the line outputs are tied to the headphones outputs, but in the case of the m903 (and m920 too), the line and headphone outputs are separate, meaning that the crossfeed circuitry is not available if you use the line outputs. For most people, this is a logical thing, because they likely use the line outputs to drive a speaker system with, but if you use an external headphone amp like me, it is nice to have crossfeed applied there as well.
What's obvious furthermore is that the m9xx is much more limited I/O-wise. In the past, the superb I/O handling of the m903 (just like the m920) was a major plus for me, but for a while now I have been using it as a USB DAC only, and now the tables have turned, as currently the smaller footprint of the m9xx is an advantage in my case. This is also one reason why I could not justify an upgrade to the m920 instead of the m9xx.
I'll be selling the m903 (offers are welcome
, because I like the DSD and crossfeed functionality of the m9xx, and do not use the I/O capabilities of the m903 anymore, but that's strictly it. Sound-quality-wise, used purely as a DAC, there's no reason to choose one over the other. As I said, I cannot comment on their headphone amp sections.
