FM reception in Los Angeles

I'm hoping that someone else in the LA area can shed some light on this problem before I start throwing money at it.

I live in the Silverlake area of Los Angeles (near downtown) on the south slope of one of the hills. I have a Magnum Dynalab ST-2 antenna in the attic connected to a MD 205 Signal Sleuth, which in turn is connected to a Rotel 940AX. I get good reception of any station I want except for KUSC 91.5, which is of course the one I listen to most. I get a good strong signal, but there is a lot of sibilance/static on the highs. This is apparent in mono mode, but is really bad in stereo mode. Fiddling with the Signal Sleuth can decrease but not eliminate this noise.

I'm beginning to wonder if this is not so much a problem with my setup or location as a problem with the signal itself. Does anyone else have trouble getting clear reception from this station? Would I be better off with a more sophisticated tuner? Can anyone give me some ideas as to how I might clean up this signal? Thanks.
I don't personally listen to KUSC so much, but I worked at UCLA's radio station in college and our signal was unbelievably weak. I listen to KXLU a lot and I always notice that their signal is weaker and more static filled than the others, so I think your guess is right, it's the signal that is the problem. As for how to make it better, I'm not really sure. Good luck!
i listen to kusc a lot, and it sounds just fine. i use an outdoor antenna, the winegard, and i think that it helps a lot, especially in an area like LA with a crowded fm dial and reflections off of the mountains. fm tuner is fanfare ft-1a, a very nice unit.
Thanks for the suggestions.
If the signal strength is high and you're experiencing treble silibance, then the problem is multipath. Try re-orienting the antenna. Hopefully you'll find a position that eliminates it.