The 990 won't send the LFE (Low Frequency Effects)signal to the sub if your front speakers are set to large. It assumes if the front speakers are large that all the low freq stuff is going to them. I would suggest playing with the crossover and moving it up to 80 hz to let the sub do the hard work and save the power of your amp for the main speakers. The sub is designed for the low freq high power stuff so why task the mains with it. Although I am sure your mains do well at 40 hz they are actually working their hardest at that point. If you use the sub for everything less than 80 hz there is more power available for your mains to work where they are more well suited. Change your mains to small and let the 990 do the distribution.