If I were you I would locate the source of that high freq. An eq shouldn't be the only option you have. I ran an SB 5.1 live w/Alpine 510 EQ 48kHz optical input to changer port and then out to multiple amps - extremely quiet noise floor in my '65 Mustang. Now I have Firewire output to M-Audio 1814 which in turn is Optical input to changer port on the 510 and very little noise and only on right channel when car is running - not sure why the noise but very faint and does not increase with volume.
So, make sure your SB is configured with the right freq for the fibre optic ie 44.1/48 kHz. DON'T use cheap fiber optic cable - can cause problems like data loss and maybe the problem your having. I did a fiber optic interface to a stock MB 500 system for an ipod and the fiber optic ends weren't even flat and smooth/polished and there were all sorts of noises in there. When I filed it flat and polished it the system worked fine. When I first set up the new system in my car I ran a good quality fiber optic to the trunk - as I bought a cheapie and didn't like what I saw once out of the package. Anyways, some weeks latter I tried using it to connect 1814 to 510 in the trunk cause that's all I had that night and it wouldn't even pass a signal = no sound!
So make sure you have the best fiber optic cable you can afford and see if that works.
If you have any more information let me know.