QUESTION: What would a basic JSFX speaker crossfeed simulation plugin for headphones to mix/simulate studio speakers look like?

ANSWER: A basic JSFX speaker crossfeed simulation plugin for headphones would involve creating a script that can be applied to stereo audio to simulate the sound of speakers. Here is an example of what such a plugin might look like:

/* JSFX Speaker Crossfeed Simulation Plugin */

// Define the plugin parameters
param wet;
param dry;

// Initialize the plugin
init() {
    wet = 0.5;
    dry = 0.5;
}

// Process the audio
process(sampleL, sampleR) {
    // Split the audio into left and right channels
    var left = sampleL * dry;
    var right = sampleR * dry;

    // Apply a low-pass filter to simulate the distance of the speakers
    left = lowpass(left);
    right = lowpass(right);

    // Apply a delay to simulate the time it takes for sound to reach the other ear
    left += delay(right, 0.05);
    right += delay(left, 0.05);

    // Combine the processed channels and apply wet/dry mix
    var outputL = (left * wet) + (sampleL * (1.0 - wet));
    var outputR = (right * wet) + (sampleR * (1.0 - wet));

    return outputL, outputR;
}

function lowpass(sample) {
    // Code for a low-pass filter goes here
}

function delay(sample, time) {
    // Code for a delay effect goes here
}

The above code defines a JSFX plugin with two parameters, "wet" and "dry". The "init()" function initializes the wet/dry mix to 0.5, meaning that half of the audio will be the processed audio, and half will be the original audio. The "process(sampleL, sampleR)" function is called for each sample of audio that is processed by the plugin, and it splits the audio into left and right channels, and then applies a low-pass filter to simulate the distance of the speakers, applies a delay to simulate the time it takes for sound to reach the other ear, and then combines the processed channels and applies wet/dry mix.

Keep in mind that this is a basic example and it would need to be incorporated into a larger program or software that uses JSFX in order to be fully functional, also some function like lowpass, delay and others that are used here need to be implemented in your script to work properly.