Can I add an outboard Equalizer to my music streamer ?

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...