Disclaimer: This is Danganronpa's third pair, and I still have three more to go after this. As I am trying to analyze everything as fast as I possibly can, please bear with me for just a few more weeks! :)
Sennheiser HD280 PRO is a closed-air/closed-back circumarual headphone, which is specifically made for a studio monitoring purpose.
PRO: The frequency response loosely follows the diffuse-field reference target, which is a prerequisite for a studio-monitoring headphone, according to ITU.
CON: Channel matching is slightly off & The level of harmonic distortion is quite high in the low frequency range.
ON SECOND THOUGHT #1: There are two acoustic vents located on the rear part of HD280's housing. Blocking these will boost the mid-bass about a few decibels.
ON SECOND THOUGHT #1: Not only the frequency response of HD280 PRO is far from the diffuse-field target, it also doesn't quite match the Hi-Fi reference of Harman International.
G. Lorho, “Subjective Evaluation of Headphone Target Frequency Responses”, J. Audio Eng. Soc., preprint 7770, (2009 May).