All of the source material comes from recordings of various devices feeding back into themselves either electrically or acoustically. No software based speaker emulators or distortions were used in the creation of these sounds.
echo | collective covered all of the classic forms of feedback including an SM58 through a PA, an electric guitar through a tube amped miked with an SM57 and a Royer 121, a set of Sony headphones feeding into themselves, and a pair of Uniden walkie talkies with both mics open too close to one another.
They also did a series of line level loops with devices like a SansAmp bass driver DI, a Danelectro Honeytone amp, and a Roland Cube amp. The line level loops were also run through guitar pedals and pitch/delay plugins to further bend and distort the looped signals.
Each of the samples are fully embedded with Soundminer metadata. The metadata also includes spectrograms of each individual sound to make combing through the sounds that much quicker. Spectrograms were created using iZotope RX3.
From all of this the raw material echo | collective built a cool Kontakt interface filled to the top with cool sounds and noises.
Check out the demos and the user guide for a detailed look: http://echocollectivefx.com/collections/signal-return