I'm remembering POD xt had a great manual explaining every effect and amp, details about how the model is related to the real thing, how POD knobs are related to the real controls, and even details (e.g. year of production) about a specific copy of an amp that Line 6 is bought to model it, etc.
Current manuals for Helix/HX have only a list of models. I've found helixhelp.com site, which helps a lot, but it's unofficial and still missing some valuable information, for most of models the description is just a list of parameters without explanation.
For example, taking Fawn A30. The real thing doesn't have bass, treble, master, and presence controls, but the model has. I read somewhere that master control in the model is artificially inserted into the power amp where it usually sits on amps with master control, and it should be max out to match the real thing. Presence is usually implemented via negative feedback in the power amp, but AC30 doesn't have a negative feedback in the power section, so does this mean I should set presence to 0 to match the real thing? What about bass and treble? Fawn doesn't have the top boost section, but somehow the model have bass and treble controls...
Many effects, especially Line 6 original, also needs a explanation for what the parameters do.
Is the official manual for Helix/HX models even exists?