Quote from: martok on June 29, 2011, 02:14:41 AM

So it appears these other posts can be applied to the Linux situation as well. The current state is that the 6990 doesn't honour the memclock set with amdoverdrivectrl at 300. However, I can underclock mine as low as 755 using amdoverdrivectl and it will be honoured. Any lower and the clock jumps back to 1250. But 755 is certainly better than nothing.

Mine are set to 500 for the adapter that also handles the graphics for the monitor, and 450 for the other (3) adapters. If you play with the voltage (lower it *a bit* in AMDOverdriveCtrl), maybe you can get also in that region. I think that would lower heat production significantly.
But I'm not sure about your situation, because due to cooling issues I have to limit the core clocks significantly: 500 MHz @ 75 deg C @ fan speed 65%.
Voltage I think I put at 1.090 V.