From Mola Mola support:
There is dB’s and dBu’s. dB’s is relative to something, dBu’s is relative to an output voltage (0dBu is 0,775V). The volume control of the Tambaqui in dB’s is relative to the actual output voltage. So if you have set it to 6V, the volume in dB’s is relative to +18dBu. So 0dB volume is 18dBu.
If you have set it to 2V, 0dB is relative to 8dBu because 8dBu is the same as around 2V output.
This scale is logarithmic, so dependent on how much you would like to reduce it, is how much volume you need to lower it.
So for example if you want to output max 1V which is the same as 2dBu, you need to lower the volume by 16dB, if set to 6V. If set to 2V, you need to lower the volume by 6dB to output max 1V.