Actually, now that you mention it, it really isn't that unusual to have to tone things down a bit in a recording environment. You're in a studio - not a club with poor amplification. You don't have to bash the cymbals or the hat so hard; the mics won't have trouble hearing them. And the harder you strum that accoustic, the less tone you're going to hear from the guitar, and the more pick on string noise you're going to get, and the thinner it's going to sound.
It's kind of like an actor who's used to theater (and playing to a large audience) having to tone things down should he/she make the switch to an afternoon soap or a dramatic movie. The camera is right in front of you; you don't have to make your actions and facial expressions so large.
But getting back to the whole guitar tone thing ... I think the whole "less gain" thing is just a rule of thumb to help compensate because some people just generally get carried away with it. Most people when they practice or play out really are using too much gain even for that setting, but they just don't know it. It's like a singer who will just generally tend to want too much reverb or effects on their voice ... as a general rule, guitar players just tend to crank the gain too high because they're having Slayer/Pantera fantasies in their head or something and they always want just a little too much.
The single most important thing ... is to learn what a good tone really sounds like. Some people know what a good tone is, but most people don't have a clue. I've had guys give me the old line; "but it sounds great coming out of the amp. Why doesn't it sound that way on a recording?" Then I'll have a listen to their tone coming out of the amp and just sort of scratch my head .... Is this guy serious? Does he really think that what's coming out of his amp is actually good? Because I'm listening to it, and it sounds like dog shit ... just like it does on the recording. Why can I hear that clear as day, while the person playing is just oblivious? It's about perspective and understanding the difference between good and bad tone.
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