These are for subwoofer users. It limits the low frequency signal (below 65 Hz) going to the main speakers, thereby allowing more efficient use of the main amplifiers power output.

http://www.audioc.com/accessories/Hipass.htm

They retail for $29. and if anyone wants to keep it, they can donate the proceeds to AK.

Sounds like a Cap in the Box, only if it fits in an RCA plug you have mucho mF in a small package. Thankyou but I will pass on this one.