Do you want to adjust the Spotify sound only or adjust sound of your music streaming from your server? If you want to adjust Spotify sound and you have to use Spark, there is not much you can do on Spark. Do you have a dialog? You can easily adjust the local streaming if you use a third party server e.g. JRiver and stream the adjusted sound to Dialog via optical. You can't adjust the sound between the dialog and phantoms except via Spark.
Remote >> 2x Silver Phantoms << Eth << Dialog UPNP << JRiver MediaCenter 24 - Ubuntu 18.04 x64 [VMWare] << File server - Windows 10 x64 [Supermicro X11SSH-LN4F, Xeon E3-1425 v5, 46 Tb, 16Gb DDR4 ECC] * Spark is only used to update firmware *
