With today’s high track counts and nearly limitless power to add plug-in processing, it can be easy to lose track of gain levels, and ignore the fact that one plug-in or another may be be raising the gain or lowering the gain of an entire track. As a general rule, you want to try to keep plug-in gain as close to unity gain (in other words, the processed volume the same as the input volume) inside a plug-in. There are exceptions, of course, such as dynamics processors that are designed to reduce peak levels or to add output level during mixdown, such as limiters. This means that for plug-ins where gain is added, such as an EQ used to boost certain frequencies, you’ll need to reduce their output level to maintain unity gain.
When a plug-in is operating at unity gain, it’s easier to audition the plug-in because the volume of the bypassed and effected signals will match. Also, this means that if the signal is going into another plug-in after the one you’re working in, there is less chance of overloading the subsequent plug-in by feeding it too hot of a signal.
If you want to raise or lower the volume of the track, use the channel’s fader or a specifically dedicated volume/gain plug-in; don’t use other processors for a volume boost or cut if you can help it.