In desperate need of help

Hey all. I've been having this problem for almost a year, but it's tax time and if I have to replace something I'll have some money, so I've just started going after it aggressively. Right now I have an Eclipse 5423 HU, Diamond Audio s600s component speakers run off of a 600x2 amp (I'm not that worried about the rears as far as this is concerned). I also have an Eclipse equalizer. Here's the deal. In my old car, a 2001 Honda Prelude, I had basically the same setup except the amp was an Eclipse amp only pushing about 85w. It sounded amazing, it put others with much more expensive systems to shame. Clean and crisp up to the max volume setting. Now that I have essentially the same thing in my 350z it sounds . . . not so good. No matter what adjustments I make on the equalizer, the amp, and the head unit, there is a section of frequencies that are MUCH louder and harsher than the rest. More specifically, it's the higher mid and lower high range, if you get what I'm saying. For some reason, even with these frequencies all the way down on the EQ, they are shouting to no end. My friend is an installer at Best Buy, so he and I took the crossovers out of the doors and messed with turning the settings down on that. No luck. The only changes to be made are to lower the tweeter between +2, 0, and -2 db. They're currently on -2. So what we did was we took a pair of Kenwood components with their own crossovers and hooked them up to the amp to see if it was the speakers or something else. The Kenwoods, while a much lower quality speaker, still exhibited the same thing. Overall smootness on low mids and high highs, but the area in between was noticeably dominating the rest. For example, U2's With or Without you . . . about a minute into the song there's a whistle type noise that's in the background. This whistle is absolutely piercing. Also, Bono's voice, when he gets into his higher range, is piercing as well. Another example . . . in Breakfast at Tiffany's (kind of a fruity song, but good for tests), it sounded excellent, but when the chorus came in and there's a lot of that upper middle range, it's awful. Anyway, the Kenwoods had the same problem. So now we're down to, it's the amp or it's the head unit. The head unit does not offer a terrible amount of adjustments. It's pretty much limited to a high and sub crossover, as well as a frequency setting for the treble and bass. Both do nothing to help the problem. For this reason, I'm thinking it's the head unit. I don't think the amp, since its primary function is power, would have much to do with this, am I right? We tried the settings for the high pass but to no avail. I've talked to Diamond customer service and I have the high pass at their recommended setting. This brings me back to the head unit. The only thing I'm having a hard time with is that I had the same head unit in my Prelude and it was perfect. My girlfriend, who went from an '01 Civic to an '04 Spec V is having the same problem, except her system is the EXACT same equipment that was pulled out of her Civic and put into her Sentra, basically the same equipement as me except with a Rockford amp. Sounded great in the Civic, terrible in the Sentra. I know imaging definitely plays a part in building a system, but these are $500 a pair speakers. There's no way they should sound like this. Anyway . . . please help!!