Sony HAP-Z1ES or NAD M51?

Go to slimdevice forum and download the third part app that enables 24/192 and add other goodies onto the Touch. Then replace the 5V DC wall wart with a linear power supply, best lab grade, add a proper after market power cord / socket and it gets as good as the 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.

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