Did you ever resolve this issue?? I just had to return a brand new Navigator Reserve because of this very issue. It had the Revel Ultima upgrade, and this problem (terrible sibilance) was so bad that the audio system was literally painful to listen to -- basically any "s" in spoken material was excruciatingly harsh.
I tried several other cars on the lot with the Revel Ultima system and they all had this problem! I then tried a car with a regular Revel system (not Ultima) and it didn't have this issue. So I'm now looking for a car with the regular Revel system.
This is totally baffling to me. The Ultima system is so bad as to be basically defective in my view -- I mean, there's areas of subjective disagreement about audio quality, and then there are just obvious flaws; this is the latter. Yes, I'm an audiophile (actually, the speakers in my reference system are Revel Salon 2 Ultima 2s!); but you don't have to be to notice this issue. To be clear the quality of the Revel Ultima system is I think generally poor; the sibilance reflects way too much high end and a lack of balance. But I don't expect much from stock car stereo systems and would have kept the car if it weren't for the harshness issue which pushes it into defective territory in my view.
The only thing I can think of is that despite being "middle aged" my high frequency hearing is still completely intact (e.g., I can hear a 20kHz tone, no problem); maybe much of Lincoln's customer base as well as all of the people who designed and tested this system can't hear this problem?
What was odd is that dialing back the treble and mids didn't even seem to fix the problem! On satellite radio news the sibilance was so bad, even with treble and mids reduced, as to be simply unlistenable.
Very curious about others' experience.
