You say you have plugged a connecting cable between your B2 (in Port 14 at the back I take it!) and your Amplifier (
yamaha A-s501 ).
Is this a analogue connection or a Digital/optical connection.
Port 14 on the B2 is dual purpose, it will provide an Analogue output - ( and the level/volume of the output is controlled by the Brennan Web UI / and / or the big round nob on the front of the B2. If your connection is analogue then try turning up this volume/level) - AND a simultaneous Optical output.
If you are using a digital/optical cable (which is a jack plug at the B2 end) and a square connector for your amplifier at the other end. Make sure that the cable is in firmly at both ends - push quite hard!
The volume controls on the B2 do not change the "volume" of the digital output of the B2. Any volume adjustment is solely the responsibility of the amplifier.
In my case, my amplifier does not have an Optical input so I use a small external DAC "in the middle" of the connections
and this work very well for me.
Fred