Sony HAP-Z1ES or NAD M51?

I first replaced the wall wart with CI linear power supply.
Big improvement! As an engineer who dealt with measurements
devices and familiar with Hewlett-Packard quality, I got a
HP 62005E on EBay just for fun (many are under $100, there
are various models providing 5V, anything more than 3A is
good enough). Everything got even better, more organic. To
be fair,the power cord and plug are of better quality than
the CI stock version. I also put some very affordable Aurio
V1.2 under the Touch as vibration control.

Another example is the the Music Streamer HD that I have in
my library system, it sounds ok - as some reviewers noted -
w/o external USB power supply, but if you follow Alan
Sircom's advice and add one battery based or, best the AQVOX
isolated linear power supply for USB, you have a darn good
one.

If you go for looks, these combos looks indeed a bit messy
vs a one box solution.

I can not stress enough the importance of good, clean power.
As you can see in my system, I like tweaks in this category
which includes dedicated lines, wall sockets, power
conditioning, cables, plugs, fuses and even VA Quantum
chips. As always, YMMV.

Another vote for the M51. My source components are the NAD M50 (which is also incredible), a HD cable box and BD/DVD player. First and foremost, the DAC sounds wonderful. Very organic sounding and lots of detail. I also really like it as a volume control. I'm using it to control a Plinius SA-103 and it makes me wonder whether I need a line stage. The amount of inputs are also a big plus. I've not seen another device like this. NAD hit a home-run with this piece.

Thanks for all the feedback on the NAD, folks! Seems like a lot of fans out there....

Still curious if anyone has compared to the Sony HAP-Z1ES purely as a DAC.

I have ordered both and will let u know what I end up keeping.

To be fair, you can't really compare these two products "purely as a DAC". The Sony has no digital inputs, so you are always using it as a complete product providing file storage, playback, and DAC functionality and cannot use these functions independently. It is designed as a complete solution and not as a stand alone DAC.

Second that, if the HAP-Z1ES has digital input and a decent digital volume control, I would be all over it.