What is the master spectrum supposed to look like? I get that it’s there to help us know how the frequencies are distributed, but if everything is properly mixed, what does the master spectrum look like? I guess I just want a model so I know what I’m supposed to be going for.
It’s more a tool to see if individual instruments have the right frequencies so that they won’t clash with each other. For example, you can let a synth or a kick play on (solo) and look what frequencies are involved. With EQing you can shape the sound so that it has its own frequency range.
Edit, and the perfect shape is, ofcourse:
Are the lines drawn at a funny angle, or is this image an optical illusion too?! 
So the lows are supposed to be so high in comparison to the mids…That’s what mine look like, except my highs are way too low. I guess I’d fix that with EQ, right?
There is no perfect ‘wave’ for music in my opinion. The Fletcher and Munson diagram I posted is just to show how many dBspl to each frequency there is needed (average human) to get the same loudness experience.
That diagram essentially explains a solution to the fact that our ears perceive different frequencies differently. High frequencies seem louder to us, so we need to compensate the low end with more volume. Mid-range is a little more difficult as it’s dependent on what style you’re working in, and also quite dependent on personal preference. To answer your question, yes, you could EQ to boost the high frequencies, but only do it on the high frequency parts of the track.