What studio monitors do you use? And how/where do you have them set up? I used to use my hifi before I could afford decent studio monitors, and the results were fine, but good monitors well set up, and ones that you know how they relate to the finished product are better. You do have to reference the finished product on various systems (hifi, car stereo, iphone etc) and this might be the phenomenon you are referring to. I have to say, I increasingly use decent headphones for mixing now for convenience though.