Did you try the high level inputs in your sub?
You might want to try that as an experiment to rule out a problem with your sub out connection out of the 990. You will need to temporarily change your settings to no subwoofer on the 990. You could leave your main speakers disconnected while you try this experiment. Also, gonk has a test disc that you can download and burn that can help you with this.

You might also be surprised how much difference placement makes. My Outlaw LFM1-EX was actually exhibiting the opposite behavior (initially) - it felt weak. I moved it 6 inches closer to a side wall and everything in the room started shaking and I have had to turn down the gain quite a bit.