Hi Marius;
Most of the information in that '10B Attenuator' link is devoted to changing the individual gain of the high and low pass filters, and to the position of the gain control on the unit. However, if you look at the centre of the rear portion of the board, there are two jumpers for setting the input gain sensitivity of the 10B, and this is a much more effective way to reduce the noise floor of the system.
Depending on how the unit was configured at the factory, these jumpers may be installed across both pins, or uninstalled, in which case they would be on only one of the two pins. To reduce sensitivity, and thus noise floor, ensure that both jumpers are shorted.
Regarding the unexpected change in gain of the system, please check the gain switch on the BHA1; it's possible that it was moved to the 'high' position, which would result in a significant increase in overall gain...
As an additional troubleshooting step, please check the noise level with the preamp turned off, and the 10B Sub turned on. This can help determine whether the noise is originating with the 10B, or earlier in the chain.
Thanks,
Mike