FWIW: For my studio needs (composing orchestral-type music via Pro Tools 12, a MultiDock II, Dual ethernet, running on a extremely quiet and powerful machine in my control/recording room, with Vienna Ensemble Pro Server 6 running on a remote 2012 Mini, I went with a used nMP from
www.ibuildmacs.com It's 8-core, 32GB RAM, dual D300's. It still has about a month of AppleCare and it's been "burned-in" -I really wish that my other 2012 Mini could have handled all that I was throwing at it, but enough was enough and for my studio, this machine is great.
I looked at the z840, the Boxx, the 2012 cMP, and they all had their ups and downs. Mostly, it came down to the Xeon, noise, and me being up to my neck in Apple since 1992. I didn't really want to spend $5k on a PC, deal with Windows and still have to buy a 35-foot KVM scheme that is not VGA-based.
If you're a gamer, this is not be the machine for you. It's awesome for Pro Tools 12 and FCPX. I needed a firetruck, not a Huracan.
PS:
DaVinci Resolve now supports Open CL
and CUDA. It's now an editor that can export tp FCPX, Premiere, Avid, etc. It can export an AAF. Great news for Pro Tools users. And it's free.