It sounds like either the power supply that supplies the vibration motors, or possibly a bad connection on the wheel base side between the rim and base.
You can rule out the rims as it would be very rare to have two rims with the exact same fault and the firmware is obviously working as you can hear the motor when it's set to 40.
This would indicate the power to the motors is not sufficient, and the most probable causes would be the power supply inside the wheel base or a poor connection between the power supply and the pins on the wheel rim.
from your exp. is vibration a must and take it to service or its ffb enough. when i move over kurbs i get vibration . when i drive out of stage i get too but i dont know if it is from ffb or from vibration.
did you try your rims?
another think i notice now is that when i press the settings button, the power light on the base, turn off and turn on one time but very fast. its like there is not enough power. but i get very very strong ffb that makes me confuse about the power...