Ohms - How do they work?

[Hand slap]

A speaker will quite happily run on a square wave, so long as each half cycle is equal.
The problem is when the rail centre/balance point moves toward one of the supply rails, and introduces D.C. into the voice coil.
When the output duty cycle exceeds 50% on one supply rail (because if it happened on both the amp would blow).
At that stage your gear must be getting severely abused, so you would deserve it if something blew up anyway ...

In a perfect world the duty cycle could be 50%, once you reach the top of a cycle, negative or positive, and it is a square wave, you are now, riding on a DC level, tell the cycle, pulses the opposite direction, hence clipping.