In-ear headphones demand very high signal-to-noise ratio, but don't require high output, because the efficiency is high. Full size headphones typically have very low efficiency and the impedance is relatively very high.
To excel with both types of headphones, the HM-901 has a low-gain output and a high-gain output switch. Low gain is for the smaller type headphones, like earphones, earbuds, in-ears. The high gain is more powerful and is designed for use with full-size headphones.