Here is the frequency response measurement of the beyerdynamic Xelento compared to the Astell&Kern AKT8iE (the Xelento's semi-sibling (Fig.1): Fig.1 beyerdynamic Xelento frequency response, compared to the Astell&Kern AKT8iE (By the way, the measurements were made using the medium size Darth-Vader-helmet-looking silicone ear tips.) As you can see, this is very different from the measurement posted earlier (by someone else) in another thread.* Here are the THD measurements versus the AKT8iE (Fig.2): Fig.2 beyerdynamic Xelento total harmonic distortion (THD), compared to the Astell&Kern AKT8iE As you can see, the THD is very low for both of these models. The measurements included in this post were made on the same day using: GRAS 45CA Headphone Test Fixture with: GRAS RA0401 High Resolution Ear Simulators GRAS KB5010/KB5011 Anthropometric Pinnae for 45CA Audio Precision APx555 Audio Analyzer THX AAA-888 audio measurement headphone amplifier (custom-built by THX) Herzan custom acoustic and vibration isolation enclosure * I suspect that the previously posted measurement did not mimic the input and transfer impedance of a human ear with an ear simulator/coupler -- an apparatus that connects the DUT (device under test) to a microphone in such a way that the working load on the DUT is the same as if used on a real ear . Modeling the input impedance of the human ear becomes increasingly important the higher the acoustic output impedance of the DUT . Also, the need for modeling the correct impedance increases with the proximity of the DUT to the ear . It is for these reasons we use ear simulators for measuring headphones. I should also note that we are currently using a new GRAS High Resolution Ear Simulator (GRAS RA0401), released in 2017, with key improvements versus a standard IEC 60318-4 ear simulator. The standard IEC 60318-4 (former IEC 60711) ear simulator was designed in the early 1980's and mimics the input and transfer impedance of a human ear. While the input impedance was based on measurements on human subjects, the transfer impedance was based on the assumption that the ear canal is a simple cylindrical volume with a hard termination. The new GRAS High Resolution Ear Simulator still complies with the IEC 60318-4 tolerance band (which is specified up to 10 kHz), but with significantly improved performance above 10 kHz. From 10 kHz to 20 kHz the transfer impedance is within +/- 2.2 dB, resulting in much improved repeatability and more realistic THD measurements . We are also using a new, more human-like anthropometric measurement pinna by GRAS. The new pinna is based on 300+ 3D scans of human ear canals, designed to be closer to the human ear, with important changes to the pinna, concha and ear canal (versus previous measurement pinnae). You can see a short video about this here: GRAS Anthropometric Pinna You can find out more about the measurement lab at Head-Fi HQ at the following link: Head-Fi Audio Measurement Lab  Brüel, P. V., Frederiksen, E., Mathiasen, H., Rasmussen, G., and Sigh, E. (1976). "Investigations of a new insert earphone coupler," Part I in "Impedance of Real and Artificial Ears," Brüel and Kjær report.  Brüel & Kjær, "Measuring Human Audio Perception," presented at the 2018 ALMA International Symposium & Expo (AISE).  Wille, M. (2017). "High Resolution Ear Simulator," GRAS Sound & Vibration white paper.