Can I add an outboard Equalizer to my music streamer ?

all the music on Spotify is heavily compressed...though the Schiit equalizer is inexpensive and returnable...

I'm not sure an equalizer will do what you're looking for. An equalizer will vary the output in the frequency ranges, meaning it will increase or decrease the volume in one or more of the frequency ranges it's designed to affect but it won't restore detail within those ranges that has been deleted in the lossy compression process that's used to create the 320K Spotify files.

Maybe ditch Spotify for a better sounding service like Tidal or Qobuz rather than getting on EQ?

An equalizer won't solve the lack of dynamics that is the hallmark of compressed files.

Dump Spotify and subscribe to Qobuz or Tidal.