[Kon Peki]

I mostly listen to Spotify 320 kbps and have read that because Spotify doesn't use WASAPI / ASIO, Windows Mixer will alter the Spotify output, potentially affecting audio quality.
 
However, I also read that with more recent versions (Vista and newer) of Windows, Windows Mixer will only affect output if there is a difference in sample rate between what you are playing and what is designated for the mixer output.  Since Spotify is mostly 44100 Hz, and my Schiit USB Audio Device is set to 16 bit, 44100 Hz in the Windows Speaker Properties, does that mean I don't have to worry about Windows degrading my quality in those cases?

 

[fluidz]

I find that the sound playing directly from Spotify app is softer than Fidelify, to describe it would be it sounds like a slight treble rolloff.  This is with Spotify and Windows 10 at max volume.
 
I now use Fidelify with Asio as I find it appears to have a slightly higher resolution.

 

[yay101]

Correct, as the name suggests the mixer will only resample if there is a difference in bit depth or sample rate. It will however also resample anytime mixing is required aka when any 2 or more sounds are playing at the same time on your system.

 

[Roseval]

To the best of my knowledge, the mixer is always active.
Any incoming stream is converted to 32 bit float regardless.
It will be converted back to integer as this is what DACs do expect  and dithered in the process
 
Your best bet is to set the DAC to 24, this will force all the dithering down to -144 dBFS instead of -96.
Sample rate of course should match the sample rate of the stream as the Win re-sampler does a bad job
http://archimago.blogspot.nl/2015/11/measurements-windows-10-audio-stack.html

 

[Kon Peki]

If I switch to a Mac, will that eliminate these concerns or does Apple have its own resampling process for audio output?

 

[yay101]

All platforms do.

Windows = kmix
OSX = whatever they call theirs
Linux GNU = pulse audio

They all also have ways around it.