AFAIK most AGCs (the AGCs that have crossed my way and as far as I remember) work this way:
The level of the input signal is detected and if it is below a given value the AGC applies gain untill the desired level is reached.
If the input level is above the given value the AGC applies negative gain (reduction).
And I remember when I first tried to understand and use Stereo Tool's AGC I had the problem, that it seemed as if the AGC was dead (not working at all).
My problem was:
I have tried to use the AGC only (without any Pre Amp !) by pressing the reset button and then enabled the AGC only and played especially tracks with extreme low volume.
And what happened (happens) ?
Nothing.
The Stereo Tool AGC can reduce the input level (fed by Pre Amp gain) and later raise the level again if needed.
But it cannot raise a low input level (and never above the level of the output from Pre Amp) !
It only reacts of the Pre Amp gain.
So there is always this extra step of applying gain with Pre Amp independant of the input level of the source.
Massive gain is applied even for very loud signals and in next step the AGC reduces it again (this happens 90 percent of the time).
Isn't this an unnecessary extra stage of compression that better should be avoided and maybe the root of all evil ?
So instead of this Pre Amp AGC combo only use another type of AGC ?
An AGC with built-in adjustable target level and gain rise as first stage for the input (now we have only a target output level) ?
If this is all naive nonsense (from a person that do not like AGCs

), then forget about this.