It seems the culprit of this issue is a bug in the subharmonic version 12.1.
Whenever you make a setting with the input frequency set to anything OTHER than 40-60Hz, e.g. 120-180Hz, and you deactivate -> activate the plugin, or even close and reopen the session(!), the input range is set to 40-60Hz. The input range highlight does NOT show this, the 120-180 range is still highlighted orange, but the center graphic response actually shows the 40-60 range instead of your selected 120-180Hz range.
When you manually click the 120-180Hz range it switches to the intended setting.
This 40-60Hz input frequency greatly decreases the output level (and/or character) depending on the input sound.
Having just an automation breakpoint in the session does not fix this. The automation needs to switch range and switch back again to read the proper setting. This also means you'll always have to play through some switching automation before playback (or manually click after opening a session).
Opening a session at some random point into the timeline will result in a bugged output, until it hits some input range automation.
Workaround and compatibility with older 12.1 sessions:
When you have used a single 'set and forget' setting which is now playing way too hard (actually correct) using the 12.7 version, you can set the input frequency range to 40-60Hz for the entire session to recreate a similar response you had before updating to version 12.7. Please note this output is not _exactly_ the same as the bugged 12.1 version. When testing using a sine wave I had to add about 4dB to the output trim (or the drive). Though when using a pink noise no added gain was needed, just setting the input to 40-60Hz resulted in a similar output as the bugged 12.1 subharmonic.
This issue has been fixed using Pro Subharmonic version 12.7 (tested on Protools HD 12.8.3). I will soon test PT 2018.7 and the latest (18.1?) Pro Subharmonic.
So yes the 12.7 version can sound louder, but it's actually the correct level.
Also, we need release notes when plugin updates are released
