The center is like the master.
It can be set by changing the distance AND there is a switch on the rear of the beolab 7-4 to adjust the distance to a wall. You might want to check this out.
The position switch is to be set to F (free) when the distance of the frontgrill to the wall is more than 50cm. Set it to W (wall) when the distance is less than 50cm. The switch is below the power cable plug. From the rear: F is left postition, W is right position.
After the center is configured correctly (distance to you and distance to the wall) you can adjust the volume of all other speakers (assuming, the distance is set correctly).