Lowering volume on mass

The problem there is that this will only work for the channels that already have automation and it sounds like the OP is saying only SOME of his channels already have automation.

I think using ‘Trim Automation’ might be a better solution…

In the Mix Console, enable Q-Link
Select all the channels you want to change.
Enable Write Automation for all channels.
Open the Automation Panel (in the Project menu).
In the Automation Panel, enable ‘To Start’ and ‘To End’ and ‘Trim’.
Now, if you move any fader by say -6dB, all of your channels will have their volume reduced by 6dB, including any automation curves that you have already programmed.