As someone who owns the D8000 and
reviewed the Pro version in a comparison, I wouldn’t characterize the Pro as better, but different. The reason for the $500 difference in retail is due to the inclusion of the final Silver OFC cable with the Pro. As an aside, I do strongly recommend the Silver cable since it does improve the sound noticeably, in my view.
As many know, the D8000 was not designed for heavy bass music played at loud levels since the drivers hit the magnets causing distortion. The Pro was introduced to remedy this situation, especially for sound engineers, hence the Pro nomenclature. Final changed the tuning with the Pro where the midrange was bumped up in order to increase the perceived loudness of play thus reducing the incident of the drivers hitting the magnets with bassy music, and if you’d like a more prominent midrange, then you should consider the Pro.
As to your comment about the D8000 sounding great at low volume levels, I fully agree. The Pro sounds great at low levels too.