HD600 and HD650 are both surprisingly easy to drive. They don't need a huge amount of either voltage or current. They are actually pretty sensitive despite the 300ohm impedance. The A5 will put out 150 mW into 300 ohms at a peak to peak voltage of 15 Vp-p and 250 mA. E12 has similar power output. Its actually more powerful than the O2 amp, and look how many people were using that as desktop amp to drive their HD600/650.