The Duo GT comes in 2 versions - Hybrid and fully-active. The latter has the iTron amp system built in, so no separate power amp is required. The hybrid is similar in design to your exising Uno and the Mezzo XD, apart from bigger and better drivers, DSP, etc.
I started my Avantgarde adventure with new Unos in 2002 (with SUB225s) and upgraded to used 2006-vintage Duos and more recently to the Duo XD. Unfortunately, living in the UK, it seems impossible to audition any of the G3 speakers. My next upgrade is likely to be the iTron Duo GT.
When I first read Roy Gregory's initial "review" (more a preview in fact), he disparaged earlier Duos by saying they had poor bass to mid/top integration. He repeats this in his more thorough review of the Duo GT - that I haven't yet read in full! On another high-end forum, I responded to his preview / review with this as my first paragraph:
"When Gregory describes in unflattering terms "previous generation Duos", he seems to be talking about ones built long ago that have separate mid and top horn enclosures, in fact probably pre-Omega. For the last 2 versions, the Duo has the top driver built into the main enclosure and the latest XD version has DSP to allow room correction for the bass. He claims that these earlier versions lack integration between the bass and the horn section. I think he's well out of touch because every other reviewer seems to think that problem was long since fixed by Avantgarde. The XD version has incredibly versatile settings to allow a seamless transfer, even by offering numerous different crossover technologies (Butterworth, Bessel, Linkwitz-Riley) and comprehensive frequency response adjustment possibilities."
The Duo XD is a great speaker and I find little to criticise apart from the rather too complex XD software. Looking at the GT manual, it appears that the new DSP has been much simplified compared with XD - no bad thing. I would consider an upgrade to the Duo GT, but not to the Mezzo XD (too similar to what I already have), or the Mezzo GT that seems disproportionally more costly.