I was going to really be disappointed in Martin Logan if they had actually messed that up (home theater sub has to have a way to disable its crossovers), so I grabbed the Descent i manual . At least on the Descent i, the "LFE in" connection on the sub bypasses the low pass crossover. Then I realized that you have the original Descent (not the Descent i, which has a 50Hz filter and a couple other controls that you didn't mention having on your sub), so I chased down that manual . Sure enough, page 7 confirms that the LFE input still bypasses the low pass crossover (whew!). I'd recommend connecting the 990's subwoofer output to that input. ML also suggests using that 25Hz filter to tweak the sub if you are having trouble getting the sub and mains to blend together well (either too little bass or too much) - if you're getting "boomy" bass, you may need to try dialing the 25Hz filter down a bit. You might also tinker with the crossover settings in the 990.