I suspect you just aren't used to the vertical cab design. Try running your lows and low-mids at 2-oclock, everything else flat. Turn off the LF filter as well. See how that soundsI think your post would be helpful and informative in this context. I bought a new GBE600 a few years ago, used it with various cabs, Avatar, SWR. Last year bought a new GB212T used it with the 600, then I bought a used GB750 and then another 212T. At first my club gigs only required that I bring one cab and either the 600 or 750, depending on what I felt like using. But at the last couple gigs,played bigger rooms, so I got to bring out 2 cabs with the 750. It was def loud enough, had the cabs stacked vertically but I am not getting what I feel is a big bottom like I used to get when I ran even the 600 thru my Avatar B210/B212 or SWR 410/115 cabs. Are the GB cabs tuned for more midrange? Am I expecting too much from 12" speakers. Oh I get no PA support ever, so I gotta bring the heat. But maybe the GB610cab would serve me better, also I am still gettting used to the 750 EQ settings, so maybe I'll find my sound. I have the 750 with BBE Processor in a 3 space rack, a little hefty but the cabs are nice and light its prolly one of the smallest footprint setups Ive ever used. Overall Im diggin the rig so far.
Also, as an experiement, try laying both 212 on their side, and stacking them. That should prove to you VERY quickly that it isn't the 12's that are the problem, but your ears probably need to be retrained into what they think they like.