All comments are to be read as "my opinion" rather than fact!
Well the short answer is the 800d3s are far more suitable for my ears than the Carbon 8s.
The Carbon 8s it seems did not last very long in the market place before being replaced and it is not really a Proac sounding speaker. I am a Proac fan and my first speakers in the 70's were the EBS which I still have and sound real super.
I have or have had 2.5,3.5,3.8,d15,d28 and d80. I am collecting d100s this weekend.
I am relatively disappointed with the Carbon 8s which I now have as surrounds to my 800d3s. They are a fine speaker but I bought them on reputation alone. I am more impressed with the d80 which suits my ears better and which at present sit in my second system as fronts. The d100 will soon go fronts and the d80s as centre and one surround, with the other surround being a spare 3.8 (which I find to be very good too)
The 800d3 and Carbon 8 are quite different sounds. I had the 802d2 and moved them on for the usual reasons to replace them with the Carbon 8.
I will prefer the d100 and d80 to the Carbon 8 as the Carbon 8 does not seem so fresh or airy as the others, hence probably ideal for a contrast to the 800d3 in the same room.
Probably the 800d3 punches above its weight price wise too. I went to the DCS factory in England to hear the Vivaldi equipment, which was quite impressive. The point of this is that their demo speakers were £140k Wilsons, and I would be pushed to hear any worthwhile price difference between these and the 800d3.
As for the now quite old comment above about measurements for the d3s and therefore do not waste your money and buy Wilsons instead - well that is why you NEVER listen to an engineer for advice on hi fi kit! I bet he has never heard the d3s, so another example of a posting of a useless and irritating comment, especially as I HAVE heard the Wilsons!