Unsound, its not. Almost any service shop would have such gear though, you could easily set up this test at almost any one of them simply because the speaker and the amp really are of no consequence in the test- the results are that profound.
How the human ear detects volume is a fundamental rule of human hearing- the 5th, 7th and 9th harmonics are the cues that tell us how loud a sound is. If these cues are distorted (enhanced) even by slight amounts, we hear it. Bright, hard, sheen, clinical, brittle, etc. are all words that audiophiles use to describe enhancement of these harmonics by very slight amounts; 1/100th of a percent is easily heard.
How the human ear detects volume is a fundamental rule of human hearing- the 5th, 7th and 9th harmonics are the cues that tell us how loud a sound is. If these cues are distorted (enhanced) even by slight amounts, we hear it. Bright, hard, sheen, clinical, brittle, etc. are all words that audiophiles use to describe enhancement of these harmonics by very slight amounts; 1/100th of a percent is easily heard.

