Is this a normal noise? I used to have a Paradigm PW2200 and I never heard it make this sound before.
You shouldn't need a highpass for a properly built sealed sub. There is a way to see how flat the freq response is for the sub out on the OPPO. All you need is a computer with a line input. Connect the OPPOs SUB output to the computers line input and run a sweep from 250hz to 1hz. I don't think the OPPO would be boosting any low frequencies, doesn't make sense.
Also, what movie scenes are you playing that are causing these sounds? Do you have a spl meter?
Have you tried connecting the sub directly to the OPPO? I think you said you had a preamp before the sub. If you do try this, make sure the gain on the sub is at its lowest settings then bump it up a tiny bit till you get an acceptable volume.