Hold on, you do realize its the same headphone with the same drivers, except that the Pro's clamp a little harder..? So how is the Premium more refined?
Implementation is everything, and the enclosure on the Pro is a lot different than the one on the Premium. Something can use the same driver as something else, yet sound wildly different.
Note the stock Fostex T50RP compared to (insert XYZ T50RP mod here).