I think the crux of this discussion is missing the main point...
Digital audio will come into your home, from the Internet, fully encoded as a digital file. It will come over crappy wires, and crappy servers, and still make it to your home as a perfect digital representation of the audio.
Then it goes into your Roku which does ABSOLUTELY NOTHING to the audio. It passes it on to your A/V receiver.
I can't imagine that the Roku isn't capable of full CD sound, if not even better.
They have detailed specifications on formats supported right here:
Now, instead of worrying about how good the audio from your Roku is, as it is PERFECT, you should look at the source, and your actual decoder (the receiver). A high quality A/V receiver with higher level DACs in it will be capable of giving you the best possible sound for the money.
A home theater PC (HTPC) is very capable, but may be much more of a headache than you would like to deal with, and you will not necessarily have audiophile grade design from top to bottom. Even with good parts in it, a good PC still is far more likely to have electrical cross talk than a purpose built piece of audio gear with decades of experience (like a Marantz pre-amp).
The OTHER half is your source. As mentioned, you aren't even using the best quality audio from the provider you are using. You have to look into the services you are using to find out what format they are using to send you audio. Is it MP3, FLAC, ALAC, and at what bit rate? If you want better audio, you need to start with better audio. Then see how much of an improvement you get.
Bluesound is widely regarded as one of the best ways to get streaming audio. Yet, I don't believe they offer a
10 foot interface.