Hello, I also have this Bluetooth transmitter, but I bought the Avantree afterwards, the sound with the Hama was not good. The Avantree has a great sound with aptx and it connects very quickly with the headphones. Highly recommended. Avantree Audikast Bluetooth aptX Transmitter

Regarding your question, you can certainly adjust the volume directly on your Bose headphones.

Greetings Thomas

the headphones do not change the volume of the Hama, probably only their own.