I’ve had quite a few headphone amplifiers, some of them rated as high as 4W @ 16 Ohms (Burson Soloist) while some rated as little as 10mW (EARMAX Pro), and many in between. I’ve also had many headphones (HE6, HE560, HE500, LCD2, HD800, HD650, HD600, T1, T5p, DT880/600Ohm, TH900, TH600, D7000...

My friend in China helped me to buy an HM-901 about 4 months back, I was thrilled when I heard it the first time, even bought a pair of Sennheiser IE800 to go with it. Later on I added the balanced amp card together with an RE-600, so I'd be able to use the HM-900 to its full (balanced)...