Yes, the FTC Amplifier Rule is more about truth in labeling than a standardized testing methodology.
Reading the actual rule. It seems that he "warm up" is done with a sinusoidal signal of 1 kHz, at 1/8 the rated power for an hour. Then the 5 minute test starts. Because I've always seen the test listed as 20 to 20k Hz, I assumed that was the requirement, but the only requirement is that the test signal be across the band for which the amplifier is rated.
The reason many read the rule as all channels driven is because of these phrases, "...measured with all associated channels fully driven to rated per channel power...", and, "...only those channels dedicated to the same audio frequency spectrum should be considered associated channels that need be fully driven simultaneously to rated per channel power." The second part meaning you don't have the drive the sub at the same time. But a manufacturer could say that the surround and center are not dedicated to the same audio frequency.
