another review - http://www.headphoneinfo.com/content/yamaha-pro-500-review
check out "the science" section for some graphs.
The CSD plot at the above link shows a pretty fast response above 1kHz, in line with some of the better headphones in the CSD thread:
Interestingly, there is definitely more "hang" in the Pro500 response for low frequencies, somewhat like a loudspeaker would in a smallish room.
This headphone has always sounded closer to a loudspeaker to me as compared to other headphones (which is a good thing). The CSD info seems to confirm that.