nv+ v1 or v2 or 104?

To answer your original questions [changing them as needed to the ultra or pro]

yorkman wrote:
1) What is the max. storage volume I can create with the ultra...

There is one 4 TB drive on the HCL, and there are others that work [though you risk losing support if you use them]. So you can get 4x4TB. Your 3TB RED drives will certainly work.

yorkman wrote:
http://www.smallnetbuilder.com/nas/nas-reviews/31272-netgear-readynas-ultra-4-reviewed ...says max. is 2TB drives...

That review was published before 3TB internal drives were available for sale, so it was true at the time. However, the ultra firmware was upgraded to support the larger drives.

yorkman wrote:
2) I currently have an X-RAID2 volume in my nv+ v2. Can I move the 3 red drives I mentioned from the nv+ v2 to the ultra without losing data?

No. You can't migrate them to any other NAS platform w/o losing data.

yorkman wrote:
3) The nv+ v2 has support for Active Directory users & groups. Does the ultra have it too?...

No. Look for a pro-4 if you need that. Also, the pro-4 has a 5 year hardware warranty and is faster than the ultra.

Basically, with ReadyNAS you can get an RN1xx/3xx/5xx which are the going-forward lines, or you can get the older Ultra or Pro (while they are still in inventory). The Ultra/Pro would have all the features you reference, but has a dated user interface, and is mature [that is, don't expect it to ever do any more]. The OS6 on the other hand is brand-new, and the firmware still needs some work.

Be more careful on the model names though, otherwise you might not get what you expect.