Everything else being equal, should I expect better Spotify quality through wifi or USB?

Given your set up, I don't think you'll hear a difference between ethernet and wifi and either should sound better than USB given the same quality content. 

It depends on location of Wi-Fi receiver. If it is inside of the DAC then USB and Wi-Fi should be equal since you’re not transferring music but data. USB might inject computer noise into your DAC while Wi-Fi can have dropouts. I had to change my Wi-Fi to 5GHz band because of that.

kijanki.  Great information.  Thank you.  Is there an easy way to tell what band my wifi is operating on?

I use dual band router and dual band receiver (Airport Express).  My router has different IDs for different bands.  On my computer I select one corresponding to 5GHz (can be checked in router setup).  It also depends what do you use as receiver.  Many products, like Airport Express, don't allow direct connection (ad hock) and require router, while others might work direct.

Thanks, Dougmint for raising this question; I have the same one.

I've recently been experimenting with Tidal, and am connecting to my preamp via a fairly generic $25 bluetooth 'adapter' from Amazon. I'm guessing this contains a wifi receiver plus a DAC. The sound, while not bad is a bit plasticky, like Muzak.

I've been wondering if the weak link is the DAC or the fact that it's bluetooth (as opposed to a hard-wired connection). Could I improve the sound by getting a DAC that accepts a USB? or Coax? Or would it just depend on the DAC?

Gasbose