What player to stream Amazon HD with?

That looks like a great solution! 

The Nativsound is likely an excellent platform.  I would ask if they use a linear power supply.  It may be one of the better platforms because of a good digital clock (looks like a TCXO).  But it's expensive, yes.

For a much cheaper alternative, you can google amazon for "android 10 tv".  There are several android tv set-top boxes running the latest Android 10 OS available for $50-70.  Then get a linear power supply to power it and you'll have a really excellent solution.  You can download the standard "Amazon Music" googleplay app to stream Amazon HD.

Alternative, you could get a Roku and upgrade it with linear power supply if you want something more well know and/or supported.   I would just make sure the Amazon Music app is available for the Roku.

These both output HDMI, which you can connect directly to your Oppo HDMI input (unless you're using it for something else).

You might want to upgrade your Oppo 105 to a linear power supply as well (available from oppomod website).  The 105 is describe as having a linear power supply inside, but that only is used for the analog output boards.  There is still a switching power supply that powers all digital sections.l

A little more on the Amazon HD and Cocktail. I am getting 44.1 32B AAC. Like I said earlier, it is much better than no HD but not as good as it could be. Cocktail says they hope to get amazon HD mid 2021. I am now trying Qobuz. Looks like a lot more HD to choose from than Amazon. Getting lots of 24 bit some at 44.1, most at 96 and some at 192. Even the 44.1/16 sounds real good.

Also the usb connection between my Cocktail and Mcintoish is working correctly now. The DA-1 with newer firmware has an auto mute feature that can be turned off and files no longer start a moment late. I think this is just a USB thing and not necessarily unique to the Cocktail or the Mc.

I just checked and my Nvidia shield pro 2019 has an usb out, It's has Amazon standard but hope they will upgrade to HD as that will be a great solution , it's the main reason I got an outboard dac to toy with.