Good hardware, really poor software. I've owned two Denon HEOS Amp HS2, and I've sold both of them. The app would stop working and freeze. I had to disconnect and reconnect the amp power cord in order to gain access to it again.
I am now using a BlueSound PowerNode amp, and the has the same issues as the HEOS Amp HS2.
For what these units cost, I rather recommend buying a Networked AVR. My Denon X4200 that I've got in my home theater system works much better than a stand alone music streamer.
The good things about the Denon HEOS Amp:
1) It supports a lot of music providers.
2) It's sound good.
3) It's awesome when it works.
1) Software. When the app doesn't and freezes.
2) Disconnecting power from the unit to regain control.
3) Having to run initial set up multiple times when re-installing the app.
The Denon and Bluesound have a better DAC than Sonos. However, I don't think most people will notice the sound difference.