I have a box with 2x6990 and 1x6970 on stock BIOS, running at:
500MHz core / 450MHz memory @ 1.0V for the 6990s
500/400 @1.0V on the 6970
1120MH/s - 480W at the wall. Oh, and having fans running below 40% is nice!
There is another small voltage step to 0.95V if you drop all the way down to 275/150, but I saw not reason to run that low.
At such a low clocks you should be able to reduce the voltage much, much more.
Not sure which software will work though without BIOS flash but you can give a try to all the tools available.
For me barelyclocked works well for undervolting/overvolting all cards.
HD 6770 - works 24/7 stable @ 775/180 with 0.82V (stock was 775/1000 and 1.125V). This way temperatures decreased by 27C (the card is in a very hot place).