I did try and look at the settings in spotify but couldn't seem to fix the issue.Depending upon the source (phone) and method of connection (Bluetooth), you may be able to set the source volume to max prior to connection, which should increase the volume through the system.
Missed these other threads, my searching obviously wasn't very thorough.Yes, others have noticed and made threads about it (no solution as far as I remember):
https://www.supramkv.com/threads/bluetooth-audio-quieter-than-satellite-radio.6687/#post-101928
https://www.supramkv.com/threads/iphone-vs-sirius-volume.4965/#post-75518