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.
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 #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.