Let’s keep it short and sweet. First a video, if you prefer seeing things demonstrated step by step:
Set your interface preamp gain to 0, check your interface specs (look in the manual for “maximum instrument input level”), adjust for your particular plugin accordingly.
Below is a list of common interfaces and the corresponding plugin input level to use. These require the interface gain to be at minimum settings.
https://docs.google.com/spreadsheets/d/1bZHaapCiCg4RLIFqTS5KyUUVa4MwaqfxRCYk35Bvdrs/edit
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This will give you the correct gain response from the model for ANY guitar or bass. Single coils will produce the correct response, humbuckers will hit the amp’s front end harder, just as would happen with the real amp.
If you adjust your input level so your guitar peak levels hit a certain level, you end up making all guitars the same volume - this isn’t something we do with real amps, so it makes no sense to approach plugins like this. Matching the headroom of your interface to match the intended headroom of the plugin is the way to go, it’s how the manufacturers intend their products to be used for the most accurate results.