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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.
 
I have a set of Altec Lansing 621 2.1 speakers. I bought them about 2-3 years ago and have been VERY pleased with them! I think I paid something like 150 euros for them so they'd be about the same in $s. They have been replaced by other speakers in the mean time I'd recommend taking a look at the higher end Altec Lansing speakers!

For example:
http://www.alteclansing.com/product_details.asp?pID=FX6021
I've heard these and they sound fantastic. Maybe $300 is a little pricey but I'm sure some hunting around the net and ebay could find you a set for $250 or less.

Good luck!
 
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