Hello. I just stumbled across the forum trying to find the answer to my questions, so I figured a post couldn't hurt. I've got a problem with my system, I think it may be the head unit, it may be the amp, I don't know. First of all, here's what I have. I have an Eclipse cd-5423 HU, Diamond Audio Hex s600s components, Diamond d5 600.2 amplifier driving them, and an Eclipse 21010 equalizer. My sub is a Diamond 10c3 with a d5300.2 amp. The sub is fine, it's only the fronts I'm worried about. The problem is that there's a range of the audio signal that is much more pronounced than the rest. More spefically it's the higher mid range frequencies. A couple of examples from songs, U2's with or without you, about a minute into the song there's a whistle type noise in the background. This sound is piercing, and physically hurts the ears. On breakfast at tiffany's, the song sounds quite good until the chorus kicks in and these higher mid frequencies are just overpowering. I've talked to Diamond customer service and have the amplifier high pass set to where they suggested, and where it sounds best, but to no avail. I'm thinking it's got to have something to do with the signal between the head unit and the amp. My HU doesn't have many options as far as tuning. It's limited to a crossover for the treble and bass and also an adjustment for treble and bass frequencies. That's it, and neither do anything to help. My next step is to connect a different signal to the amplifer, but I was hoping someone could shed some light on my problem. If it could be the amp, please let me know and I'll post this in the amplifier section.