Thanks Mark, although I think you've missed a generational change in the Duo line-up.
Yes the Grosso was introduced some year back as a "better-bass" Duo by using twin 12" drivers in place of earlier twin 10" in Duos since about 2000. However, the new Duo XD uses twin 12" drivers so it now has identical drivers and amps to the Mezzo XD. The old Grosso is no longer in the AG range.
As you say, the difference between current Duo XD and Mezzo XD is the design of the bass cabinet, but not the drivers. The latter has horn loaded (not sure that's a strictly correct description) drivers so the bass should be a little better than standard Duos with their (effectively) conventional subs.
I've pretty much decided on the Duo XD as the subs are so much slimmer and would look much more acceptable in my living room as the speakers are in the middle of the room. In fact, I will follow my existing Duo modifications by eliminating the upright support "scaffolding" that I find unattractive in the Duo. I'm still working on how best to achieve this!
I see you are very much into tube amps and have those lovely twin-845 Nagra monos and the Brazillian Audiopax amps that we often saw powering AGs at shows some years back. I used to use tube amps (Art Audio PX-25 and 845 Carissa , Graaf OTL, Audio Note 300B and more recently Consonance Cyber 845 monos) but I decided 3 years ago to search out a solid state amp that sounded as good, but without the relative hassle of tube amps. I'm happy I did, but I'll look further when the new speakers are installed - though I don't think I'll be going back to tubes. I have a short list of 3 likely candidates, mostly recommended by Mezzo XD owners. Peter