Steve,
This may or may not me what you are looking for: http://www.partsexpress.com/pe/showdetl.cfm?&DID=7
This particular link takes you to an in-line 20Hz high pass filter. It appears you basically put it in-ine between your reciever and your sub-amp. Should filter out 20Hz on down w/ a 12db slope. I didn't see anything that went lower than that.
That's my 2 cents