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.

+ 1 on Qobuz. Pay for a whole year and it's even cheaper.

To give you helpful suggestions about how to improve the sound it would be useful to know what equipment is in your system now and how you're receiving the Spotify feed.

You might look into something like...

And/or...

I own 2 Schiit LOKI's   ...Absolutely love them...Thinking of getting the 8 band remote controlled Eq. They add  0 distortion but can really enhance all your source material.

Six band @mbmi, but you already knew :-)

Having the convenience of control at the sweet spot is certainly a plus (no doubt) but how often are we changing our settings once dialed in to all the various genres that one might press “play.” The only time my Loki and Lokius feel the touch of hand, on / off toggle switch. Substantial amount more $ for seated position IMO is all. It’s your money, choose wisely.

I absolutely love them too ….

@garebear will you share the components in your system.  As others have said, spotify is not great, but maybe a small investment in a streamer and streaming service would yield huge improvements.  I am happy with a used Node 2i acting as a streamer for Qobuz.  

Spotify is greatly compressed... while an equalizer or better streamer may help a little, since the source is compressed it will always sound compressed...

Lol...  Oh you said outboard...  Neve shelford channels or Manley Massive Passive...