In episode 7 of Pete Thorn’s Tone Secrets, he dives into the phenomena behind the clipping/distortion we hear when recording clean guitar tones. The concept applies to recording tube heads and becomes evident whether you’re miking up a cabinet or using a load box and IRs.
The explanation is really pretty basic. When you’ve dialed in a tone you’re digging and are playing in the room, chances are pretty damn high you’re listening off-axis, at least a few feet away. What you’re hearing in the room is quite different than what a mic 1″ from the speaker cone picks up. Simply being that close to the driver, you’ll hear a mild overdrive that isn’t readily apparent several feet away.
You can hear Pete demonstrate this, first by distant miking the amp, then close miking it, and finally using a load box with and without IRs (RIP headphone users). The major takeaway here for those new to recording is that you’re gonna hear some wild shit you haven’t before when you go sticking mics in front of things (physically or virtually). You might find that the room sound and miked sound aren’t totally vibing, so don’t be afraid to get in there and tweak ’til your heart’s content.