Actually, even Beyer states that the drivers are exactly the same in the Pro and the Premium versions.
So there is absolutely no difference in sound. The build is slightly different though and there is more clamp on the Pro version.
The higher clamping force means that the drivers are slightly closer to the ears so the sound is slightly modified this way, but not by much. Most likely, the other differences in sound that people claim to hear are caused by placebo. Also, it is possible to bend the headband to get less clamp.
IMO, if you don't mind the retro looks, the PRO is a huge bargain since it is often sold for half the price of the premium and has the exact same drivers.