I actually own 6990's, several.
If I'm a fanboy, stop being a hypocrite considering you're only looking at individual reviews while I've seen 20+ this morning, alone.
The general consesus (if you read them all) is the 590 is 10-20db quieter. There are certain situation that pushed the 20db, sure; I can think of some setups that do that but, it doesn't change the reality that if you matched db for db, the 6990 would fry.
As someone in an office with 5 580s, 12 480s, 4 6990s and soon to be 10+ 590's...I have to say thank god to the fact the 4 interns with the 6990's are trading them for 590s...So will any admin/tech/office person that has to deal with the jet engines all day...
Sorry but, most of the reviewers, agree with me.