The difference between the GMX-A and the standard ProMedia speakers is simple.
The GMX-A uses a 50W subwoofer and two 14W satellites, for a total of 78 watts. The standard ProMedia has a 130W subwoofer and two 35W satellites. I know that power isn't everything, but it helps a lot. Quite simply, the ProMedia will sound a lot better and go to a much higher volume before getting muddy or tinny sounding. Also, the ProMedias are THX-certified, while the GMX-As are not.
The GMX-A uses a 50W subwoofer and two 14W satellites, for a total of 78 watts. The standard ProMedia has a 130W subwoofer and two 35W satellites. I know that power isn't everything, but it helps a lot. Quite simply, the ProMedia will sound a lot better and go to a much higher volume before getting muddy or tinny sounding. Also, the ProMedias are THX-certified, while the GMX-As are not.