JimenezOmarDark voice is OTL (output transformer-less), a full tube amp and single-ended. This means it requires high-impedance headphones with a non-balanced connection to sound nice. The TA-20 is a fully balanced hybrid tube amp with a tube and a transistor stage. It requires a balanced connection for best results and can drive almost any pair of headphones.
If I had to guess I'd say the dark voice has more tube character, but the TA-20 is just more hi-fi, with perfect stereo separation due to separate balanced paths and no noise at all even at the maximum (very loud) level. On the ta-20 the left and right signals go through separate tubes, while on the dark voice you will get some crosstalk due to just one power tube and just one driver tube.