[He] ..used a Sunn Coliseum PA head, when it was shipped to him by accident. He claimed that this is the amp that gave him his signature sound in a Gibson interview with West.
The Sunn amplifiers that West used were of the late 1960s era and were not factory stock. The four-channel amplifier heads' preamps were wired as cascading preamps to channel one, out to the amp's power section. This is what produced the long compressed sustain and distorted overdrive of the great Mountain sound that he is well known for.
Edited to note that this version may be incorrect about the cascading pre-amps. Reliable sources report during that time it, was a bog stock PA. At 3:55 he mentions a tech doing something..
I recently read a long article about him. He was very business savvy for a musician at an early age. He always held his publishing and copyrights, and produced many of his albums from early on in his career. And he's a better player than most realize.