when you set the ring led for power state you HAVE a bar to adjust the led intensity
when you use the power led (or the ring led) for the HDD activity you DO NOT HAVE a bar to adjust the led intensity
their is no way to control the led intensity for the HDD activity
When you select HDD Activity LED as your choice for the Button LED selection, you will be presented with a Brightness selection right below. Use it to select what you want to occur.
Note also that the color you choose to use can have an effect on the apparent brightness of the LED.