How do you see the cabinet?
Just a container to hold the speaker? Or part of an instrument that makes the sound? The same wooden box, seen which way. This difference of view becomes, directly, a difference in how you make sound, and it changes even your gear choices and recording decisions. A plain-sounding theme — but think it through once and how you see an amp's sound shifts quite a bit.
The "just a box" view
Start with the naive view.
The speaker makes the sound, the cab merely holds it. So the lead of the sound is the speaker, and the box is just a container. This view is intuitive and clear. In fact, in the world of hi-fi audio, this thinking is rather correct. An audio speaker box is ideally an "accurate container" that adds as little color as possible. The box itself resonating and putting a quirk on the sound is disliked as "box resonance" that harms the original, and it's thoroughly killed — braced inside, packed with damping.
Because audio's goal is "turning the incoming signal into air, faithfully, as is." The box should quietly support and not assert itself. In this world, the enclosure should be neutral, colorless, transparent. The sound is made by the recorded signal, and the box is asked to be a transparent vessel that passes it without distortion.
The "part of an instrument" view
But a guitar amp's cabinet is built on an entirely different philosophy.
Assembled from wood, open-back or closed-back, its ring changes greatly with structure. Board thickness and material, internal volume, how the bracing is put in — such elements of the box itself resonate, and that resonance is actively built into the sound as a part of it. In the guitar-amp world, this "ring of the box" is used as a flavor, not killed. What audio tries to erase, the guitar amp makes a weapon.
So a guitar cab is designed not as an accurate container but as a resonating body for adding color. This is close to the thinking behind the body of an acoustic guitar or a violin. More than the string's vibration itself, how the body resonates it decides the instrument's sound. A guitar cab is the same: how the box colors the sound the speaker made and casts it into the air becomes that sound's individuality. Here, the cab isn't a neutral vessel. It's part of an instrument, with a clear character.
Concretely, a closed 4x12 like a Celestion-loaded Marshall 1960 has a tight low end that flies straight forward. A Jensen-loaded Fender open combo has lows that wrap around and spread, breezy and open. A large Mesa/Boogie cab pushes with a deep low end. Even the same "guitar cab," with a different combination of box structure and speaker, rings differently enough to call it another instrument.
Which way you see it changes your sound-making
These two views aren't armchair debate — they connect directly to practice.
See the cab as "just a box" and you'll care only about the speaker and the amp, thinking any box is much the same. See it as "part of an instrument" and you'll think that, even with the same amp head, which box you ring it through sways the sound itself. Not changing a single head, yet swapping the cab makes it ring like a different amp — this phenomenon many guitarists experience can only be explained by the latter view. As CSL wrote in "how much of the sound is decided by the cab," head and cab are multiplication, not addition, and the final color is largely decided by the last box and speaker.
So for guitar amps, the "part of an instrument" view fits reality overwhelmingly better. The cab isn't a neutral vessel that only carries the sound. It's a party actively making the sound. Whether you hold this recognition changes where you look when the sound won't come out right.
Because it's an "instrument," it's a variable
And the recognition that the cab is part of an instrument bounces directly back onto the work of recording.
If the cab is a party that makes the sound, then it's one of the "variables" that decide the sound. However you fix the head's distortion, change the box it passes through and the output changes. Which means: when you want to compare something else correctly — say, the difference between amp heads — moving the cab variable breaks the comparison. You can no longer separate whether it's the head's difference or the cab's. Precisely because it's part of an instrument, only by fixing it do the differences in other elements come straight into view.
This is the thinking at the root of why CSL fixes one reference cabinet. If you think the cab is "just a box, a container that doesn't matter," you record without noticing this powerful variable, and later agonize over "why don't the model-to-model comparisons line up?" Precisely because you correctly acknowledge the cab as an instrument can you make the judgment to carefully fix it — or, when the box is integral to the model (as with a combo), to record that box's sound together. Acknowledging it as an instrument comes as a set with paying careful attention to how you handle it.
In short
A cabinet is an "accurate container" in the world of audio, but in the world of guitar amps it's a "resonating body that adds color" — part of an instrument. That the same wooden box carries opposite philosophies is what's interesting. For a guitar amp, the box doesn't just carry the sound; it makes it.
And precisely because it's part of an instrument, it's a variable that decides the sound. You need to choose which box to ring it through with the same weight as choosing the amp head. The moment you treat the cab as "just a container," you unknowingly let go of half the sound and break the ground for comparison too. Treating the box as an instrument — that's the first step to making, and recording, sound accurately.