The recommended procedure, I believe, is:
1) Make a direct amp profile
2) WITHOUT changing anything on the amp or anywhere else, make a regular studio profile
3) Copy/paste the studio profile cab onto the direct amp profile (and I do believe copy/paste rather than browsing is the correct way!)
4) Hit the merge button
5) Store the profile
This profile you have just stored is a merged profile; which means the cabinet portion of this is now a "perfect representation" of the cab+mic. Merging affects the CAB PORTION ONLY, to my knowledge.
You can throw this new cab on any di profile (but DO NOT merge them; that process has already been applied to the cab), and the result SHOULD be authentic. Bear in mind that the parameters in the profile which describes amp/cab interaction is NOT authentic to this new pairing, as that information is stored in the amp part of the profile, and does not change; how big a difference this makes I don't know.