I think what you want to do will work nicely and there's no need for a streamer.. I believe that even through USB or other digital outputs the sound does get processed by the windows mixer unless you're using special drivers (it was either ASIO or WASAPI it's been a while for me
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) so in theory that would not be desirable, however I remember comparing both ways on a decent sound system and not being able to hear a difference. The laptop or HTPC is nice too because it does give you a lot of flexibility.
As for the quality of Spotify I think it gets a bad rap, streaming from your HTPC with the program at extreme quality setting is 320 Ogg Vorbis which is very good encoding .. This is not lossless, but the bits that are lost I think maybe only your dog can hear assuming your gear can even reproduce those frequencies. The difference may be in the masters used for encoding but I'm not sure about it there is very little info in the public domain. When I try tidal I could come back and tell you it is better but I'm skeptical, most of the reviews I've come across seem extremely subjective and very susceptible to the placebo effect. There is no doubt that lossless is a better format however I'm not sure it is audible, I've had a hard time picking out FLAC over 320VBR files in the past (yes the flacs were real ripped and not upconverted), none of the reviews or impressions use any kind of sound pressure level meter to do a proper comparison, they simply assume that the volume knob in the same position yields the same level but that is probably not true (the louder one almost always sounds better to most people). I would also turn off the volume normalization setting in Spotify as that can reduce the dynamic range.. another thing I'm not sure ppl do when they do their reviews and comparisons.
One more thing, I've seen mention of the Chromecast in these responses, I'm using one right now and it is convenient but I've come across a couple of limitations. #1 the stream according to Spotify is not 320 Ogg Vorbis when casting through Chromecast, it is 256 AAC.. still pretty damn good but not as good and kind of annoying considering it is supposed to handle lossless even. #2 The playlist size is limited.. listening to an album is fine but one of my lists is like 100 songs or something and it stops playing when it reaches a certain number of songs.. even more annoying, I will be ditching the Chromecast in favour of a tablet or laptop via USB like what you're looking to do.
Lastly, good job, I like that you are trying to use logic to assemble your music box
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. A lot of voodoo in the audio world.