Paul P Old sound systems and mixers wanted high impedance headphones. For my EPM6 mixer (still sold), Soundcraft recommends greater than 150 ohms. Oddly, no company under the Harmon umbrella (notably AKG) makes such headphones. So Harmon sells a product for which you have buy a competitor's headphones (lower impedance does work, it's just not recommended). I think Beyerdynamic are the only ones to still make high impedance phones.