Dodo Bird has released McDodo's Super-Volu, a plug-in that removes the change in volume caused by other plug-ins. This helps to determine the impact they have on sound character, isolated from their effect on volume.
For example: You may want to insert a tube saturation plug-in in a channel, but the plug-in changes both the sound character of the audio and its volume. This makes it hard to understand exactly what the tube effect does to the sound. You can use Super-Volu to make sure the tube effect does not change the volume of the sound, and then you can understand better what it does to the sound character.
McDodo's Super-Volu is available as a VST effect plug-in for Windows and it is free and open source.