According to the designer of XB series, 松尾伴大-san, this first generation of XB-series (XB300 to XB1000) is specifically tuned for club-goers, who are frequently exposed to fast-paced bassy dance music. In order to accommodate their musical taste, 松尾-san prudently shapes the frequency response to be heavily bass-oriented, while keeping the transient characterstic as quick as possible.
This has been realized by utilizing a pair of huge closed-air ear pads, which effectively eliminates acoustic leak from its source, thus preserving the sub-bass response largely intact.
CON: Just like any other supra aural headphones, even a slight displacement of the driver completely flips the spectral balance around. On top of that, XB300's harmonic distortion level is literally off the hook..
ON SECOND THOUGHT #1: The rear vent must remain open; Otherwise, the mid-range gets even more annoying than already is.
ON SECOND THOUGHT #2: Simply put, Sony MDR-XB300 is far away from being Hi-Fidelity.