I tried the HP100 headphone amplifiers with Sennheiser HD 580 headphones, JVC HA-D990 headphones, Etymotic Research ER-4 earphones, and assorted earbuds. The headphones themselves proved the limiting factor, not the headphone amp. Not in the Stax electrostatic realm of transparency, the HP100 is still a huge leap over the throwaway op-amps used to power headphone outputs on most consumer gear. As a preamp it?s very basic but more than adequate.