SONY MDR-7550

AFAIK SONY MDR-7550 is an international version of MDR-EX800ST, specifically tweaked for a monitoring use unlike its brothers, EX600 and EX1000, which are tuned for a personal listening.

PRO: Very linear impedance characteristic, and almost non-existent distortion figure.

CON: For some reason the channel balance is little off with this unit, as if the left channel has been slightly clogged with wax. The frequency response can be a lot more flatter IMO. 7550's high frequency extension is little too short. Not really a desirable tonal balance for monitoring purposes, as suggested by the ITU tolerance.

ON SECOND THOUGHT #1: As expected, just like MDR-EX1000, the insertion depth does not play much role here, partially due to the fact that it does not have much high frequency content to be altered with, and partially to its low acoustic output impedance.

ON SECOND THOUGHT #2: Even a 100 Ohm resistor does not alter the frequency response of MDR-7550. Don't expect much from its electric damping characteristic.

ON SECOND THOUGHT #3: There is an acoustic vent on MDR-7550's housing, working as a depressurizer, just like that of Sony Ericsson MH1C, or that of AKG K3003. Blocking the vent will greatly boost the sub-bass, along high frequency extension.

However, it is worth to note that not only the vent prevent the drivers from getting damaged, acoustic vents sometimes contribute to a better transient characteristic of an IEM within the ear canal, especially when the diaphragm is of a high compliance.