Iphone bluetooth speaker volume is way too low

the problem: The volume of my iPhone on a bluetooth speaker is way too low, I nearly can't hear anything even if I turn the volume all the way up. In fact, my iPhone alone on half volume is louder than the bluetooth connection!

details: I am using an iPhone 13 and an iPad 4 with Spotify and Youtube - all are up 2 date - on a Stockwell Marshall Speaker. If I play from the iPad at half volume on both devices (iPad & speaker) it it much louder than having iPhone and speaker pushed to the max.

What I tried already, because many articles suggested it:

• turning on/off again
• removing the case from the iPhone (someone in this forum suggested)
• updating system, updating apps
• adjusting Spotify volume level (not suggested, but seems obvious)
• changing the device type in the bluetooth menu
• turning the option "reduce loud sounds" off / on (and changing the allowed level)

the solution:

choosing the "forget device" option in the bluetooth menu and connecting again.