This is my first headphone amp purchased with my first "better quality" headphones (DT 880/600) in years.  Nothing else to compare them with as of yet. The HDP is hooked-up to a Sony all-in-one desktop computer via USB.  The preamp outputs are connected to a pair of Audioengine 5 powered speakers.  The sound through the speakers is excellent for the type of setup, but with headphones connected, it is nothing short of Hi-Fi quality.  The volume turned up all the way, there is not even a hint of distortion.  Except for a slight bit of brightness at the higher end of the frequency range, the sound is dead flat with just enough bass impact and lively midrange.  Hopefully, I will have more to comment on as I put more miles on this setup...