I have the same problem with the same unit !! (Bass)
I tried to dissamble it, but even after removing 6 screws on the bottom,
(4 visible, 2 under the fron rubber disks), I cant get anything to move in or out or whatever.
Just to update ya`ll, I got rid of the SR-30 and replaced it with a couple of sattelites and a 200 watt sub, okay its another Logitech but I trust this one! The sub is mounted differently and this ones a pressure sub instead of being ported and it whips my last sub!