Whether there's an effects-loop jack on the back of an amp. Many people never use it even when there is one; it's a plain spec.
But the presence of that loop reflects how the amp "thinks of what it is." A device complete on its own, or part of a signal path? From the loop you can read the amp's view of itself. That's what I want to talk about.
What an FX loop does
First, confirm the role of an FX loop.
The guitar signal has its distortion made inside the amp. Here a problem of order arises. Spatial effects — delay and reverb — sound more natural applied to the sound after distortion. Apply delay before distorting and each echo distorts along with the amp, crushed, losing outline and muddying. Apply it after distorting and clean echoes ride on the finished distorted sound. Distortion first, space after — this order is the basis for a natural sound.
But in the normal connection, plugging the guitar into the amp's input, all the effects come "before" the distortion. That's where the FX loop comes in. The loop is a mechanism that provides a place to insert effects "after" the amp's distortion. It pulls the space between the preamp (the distortion-making stage) and the power amp out once, inserts the spatial effects there, and returns it. It connects distortion generation and spatial addition in the correct order. A door that invites outside gear into the middle of the signal path — that's the FX loop.
The philosophy of an amp without a loop
An amp without a loop has its own thinking.
"This amp makes its sound in this one box. It doesn't assume outside gear inserted in the middle." From input to output, it completes the signal within itself. With fewer circuit junctions, extra coloration and tone loss are less likely, and the purity of the sound is easier to keep. The signal takes the shortest path and doesn't detour. A philosophy of deciding the sound entirely in one box is there.
This is common in older designs. Many vintage amps have no loop. In the era they were born, delay and reverb weren't applied deep; placing them lightly in front of the amp sufficed. The very idea of "inserting gear after the distortion" was thin. But it's not only that they lack it for being old. Some modern amps deliberately omit the loop, prioritizing purity, keeping the path short, valuing completion in the one box. The absence of a loop is design age and, at once, a statement of philosophy.
The philosophy of an amp with a loop
An amp with a loop stands on the opposite premise.
"This amp is part of a larger signal system. I make the distortion, but outside gear follows after it." The amp becomes not the terminus of the sound but one segment of the path. It assumes, from the start, combination with a rack system or a complex floor loaded with spatial effects. It handles distortion generation and leaves the finishing spatial processing to the outside — a premise of division of labor.
The loop's presence is also a declaration of "I don't complete alone." That amps from the '80s on, feature-rich and multi-channel, almost always carry a loop is because they stand on this premise. The spatial expression music demanded grew complex, beyond what the amp alone could hold. The loop is an institutionalized receptacle for opening that complexity to the outside. Ones with send/return level adjustment and serial/parallel switching are proof of a more serious design for cooperating with outside gear.
The loop is a door for the control layer to go out
Widen this viewpoint and it connects to the larger flow CSL keeps writing about.
The center that decides the sound has moved from inside the amp to the floor pedal, the rack, and then software — outward. The FX loop was one of the first doors that physically allowed that move. By opening the space after distortion to the outside, the amp accepted, at the circuit level, "entrusting part of sound-making to the outside." Without a loop, you can't even insert the rack's spatial effects at the correct position. That huge rack system held together precisely because the amp had the door of the loop.
So whether there's a loop is also an expression of how far that amp accepted the control layer's move outward. Closed and self-completing, or open and cooperating. Two small jacks on the back become the fork of that amp's view of itself — seeing itself as an independent instrument, or as part of a signal system.
A different path means a different sound
Put a recording viewpoint here.
The sound passed through a loop and the sound not passed through it are, strictly, not the same. The loop is a mechanism that inserts a circuit between preamp and power, so even with no effect inserted, passing through that circuit (a buffer or level-adjustment stage) itself can slightly change the sound. That some amps are said to thin a little just from switching the loop on is because of this. In what order, through where, did the signal come? The path is part of the sound's history.
So to keep an amp's sound, you need to be aware even of what connection it was in and through where it passed. Through the loop, or straight through. Even the same amp is a different state if the path differs. Drop the path from the record and the conditions blur, and you can't recall the same sound later. That CSL fixes the connection and records is the obvious response to this fact that "the path is part of the sound too."
In short
The presence of an FX loop reflects the amp's view of itself — a "device complete on its own" or "part of a signal system." The loop was a door that allowed the control layer to go out.
And a different path means a different sound. Whether to carry a loop is also a choice of whether the player plays that amp as an instrument complete on its own, or builds it in as part of a larger signal system.