I have also have a Koss Esp9 ( powerered by a fet amp by A 207 R Pioneer ),
and a Sennheiser HD 414 .
ESP 9 has a design issue ( little hum ia AC position )
The solution is to replace the power supply , but it's too complicated .
Better to use SE position .
HD 414 is a good " measure instrument " . Due to its very high sensitity you
can hear every little issue such noise , buzz , hum and so on .
Good for perfect set up and good sound .
Ciao
Marco