On high Z, the L2 has a Zout of something like 50 Ohms. The headphones have an impedance of 80 Ohms at 1KHz, but 300 Ohms at 50Hz. That means there is a voltage divider with the headphone Z and amp Z. So, if there is a 1V signal, at 1K about 38% (50/(50+80)) of that signal is lost to the amp's impedance, and the other 62% goes to the phones. So you are left with a 0.62V. At 50Hz, the amp is still 50 Ohms but the phones are 300 Ohms. That means they get 86% of the signal. That's nearly a 3dB boost at 50Hz. Hence too much bass.