This tends to happen to me way too often. I mix tracks so that each has the unique part of the spectrum it's prominent at. And it works pretty well, except for one major problem. After I have done all of this, I realize my guitar starts sounding really boxy. I decrease some of the 500-800 range, and then I have to deal with another problem. Now my guitars sound way more 3D, but the mix has virtually NOTHING in this range, so it doesn't sound full or warm enough, but if I increase this range on guitars, I get a very boxy sound. The cause to this is that I do not think that any instrument sounds REALLY good at this range, so I tend to either cut it out, or remove this range by serious bandpassing of pretty much everything. So really, even the waves PAZ Analyzer says this range is (pretty much) empty. What am I supposed to do here? I know I could always increase this range by a linear phase EQ on the master bus, but I don't want to use such radical measures here, I think it'd really do way more harm than good in this situation. I've heard professional mixes where this range is pretty empty as well (even going as far as the 300-1200 range), and I never thought they had the warmth I am aiming for, so even though my mix could 'work' I don't think it's the most optimum way to be.