I own the P65+ and the D3X with Zeiss glass (not limited to the 100mm F2) and have created large inkjets from both systems. While the D3X affords much more versatility, at this point I disagree that a DSLR-fashioned 30x40 inch print (3+ vertically stitched captures required to match MF sensor size) will rival a MF non stitched print. Sadly, I am not technical enough to provide a satisfactory answer but the tonality and liveliness of MF has something (more than just the lack of filter) magical. Can someone help me out here with technical support?
Are you comparing equal size files of equal FoV taken with equal focal length of lens at equal F stop? (The pixel size of the D3X is the same as that of the P65+).
If you are, and you still see a difference, I'd suggest the following reasons, because it's certainly an interesting question worth exploring. Never having used either a D3X or P65+, I perhaps shouldn't be commenting. However, I do know from personal experience with other cameras, that the RAW converter used can have a dramatic effect on factors such as vibrancy and sparkle etc, depending on how the presets have been designed and how they work in relation to certain models of cameras.
I was very disappointed when Adobe took over the RAW converter RSP. It was my favourite converter at the time, and I still have the last version of RSP, before take-over, installed on my system. This version works with my 20D and 5D RAW images, but not 40D or 50D of course.
Sometimes, there's no doubt in my mind that RSP produces a more pleasing result than the latest version of ACR with my Photoshop version of CS3E. I have gone through the exercise of trying to emulate in ACR the precise effect I get with RSP, and after a lot of stuffing around with various adjustments and trial and error, I find that I can get very close, but not exactly.
The last RAW converter I trialed was Bibble. I was surprised to find that it produced noticeably sharper results than ACR with my 20D and 5D files, a bit like the difference between having an AA filter and not having one, perhaps.
However, I soon noticed a trade-off. The extra bite in the image was due to less noise reduction. Even the lower mid-tones had more visible noise than the ACR conversion with all noise reduction (both luminance and color) at zero.
Again, after some stuffing around with the detail, sharpening, clarity and vibrance controls in ACR, I was able to fairly closely emulate the result of Bibble using ACR, but not exactly.
In the world of advertising, time is money. $50,000 is perhaps not a significant amount to pay, if one can save time in processing one's images and get a very pleasing result for the client because Phase One has tailored its Capture One software to produce outstanding results with its DBs.
Just my own observation and thoughts on the matter. I could be way out.