HI Alex:
It is possible that the attenuation is not reaching 0 as it should. We can look into this if you send us the setting of your external slew volume control. In the mean time, there are ways to alleviate this problem.
1.- Slow down the slew rate to a higher value
2.- Connect 2 of these external volume slews in series triggered by the same GPIO and using a T connection to connect them to both external slew volumes connected in series.
Please let us know the results.
Miguel