the advanced paramater AHRS_MPU6K_FILTER is not viewable in the advanced parameter list of firmware 2.9.1 and 2.9.1b
Although this parameter seems to be interesting if you have problems with alt hold caused by vibrations of the apm.
Can I add this parameter?
On the internet speaks some persons of this parameter at this firmware.

Ah, it’s INS_MPU6K_FILTER rather than AHRS_MPU6K_FILTER. With AC2.9.1b we reduced the default to 20hz. Normally with vibration it’s best to go with a physical solution rather than attempting a software solution. Some details on measuring and reducign vibration are on these wiki page: