Hi, I did test the HD-668B on my modified SMSL SD793II dac & headphone amp where the sound is great and a bit more focused than the HD-681 (but both headphones are close) After listening the HD-668B, the HD330 has too much bass with overall heavier sound          There is a lot of details, the voices shine. The bass are deep & clear. The trebles are very clear, when they are hard, they only reflect the poor quality of the amp or dac If you improve the quality of electronics, you will hear it through the HD-668 B   I am very impressed with...