Record a simple chord or chord progression and see the difference on playback of dropping tops mids? On Logic my mate showed me FAT EQ and what it could do... made a lot of difference
One method I often recommend is to get yourself a 15-band graphic EQ (hardware is best, but a plug will suffice) and a few CDs of high fidelity and of differing music genres. Rip a few songs into Logic, and play them back one at a time through the EQ with all the bands set flat. As the song plays, take one of the EQ sliders and boost it, listening to how that band sounds in each of the instruments and vocals playng. Then push that slider down, and move the next one up and listen some more. And so on down the line, mentally noting the frequency number of each band as you do it.
Do that for a half-hour a night for a week or two, then have a friend come in, put in a new song, and do the same thing with you not being able to see which band she's moving, and yo have to try and guess which band they are moving. You may not get them all exact, but you should be able to get at least close most of the time. When you can, then you'll have a much easier time listening to your recorded music and determining just what's happening frequency-wise with them.
For ukuleles what are the frequency spectrums or is that the wrong question to ask?
That depends greatly on just which key you're playing in, of course. But all instruments can use up a wide swath of the frequency spectrum not only for the fundamentals, but in resonances, harmonics, etc. Most instruments heavily overlap each other in that the are *capable* of doing. Which part of the spectrum they seem to dominate within a given song is largely determined by the songs arrangement. Which leads to...
Now could I get a little more explanation of this point please... what do you mean by supporting it with your mixing arrangements?
A whole chapter could be written on that (and I'm working on that elsewhere

), but in essence it means listening to the arrangement of the song itself, the various roles of the various tracks within the song, and then planning and executing your mix plans to support that an not compete with it.
So what are the key elements is it like chorus verse bridge etc.. or is it more than that?
You're asking some big questions that require big answers. But the short answer is that you gotta decide and interpret from listening to it what you and the song agree are the stromg elements of the song; is it the lyrics, the hook-y guitar riff, the groove of the rhythm section, the overall mood, the melody, some special instrument fills, some kind of call and response action, the virtuosity of the instrumentalist, the vocalists voice, etc., or (probably) some combination of these.
The bottom line is to listen to the song and try to figure out what it "wants"; i.e. what parts of it are it's strong points, and should be reflected as such and supported in the mix. Should the lyrics play the dominant role as in a Dylan song, or is it the virtuosity on the uke like in a Jake Shimabukuro recording, or maybe the guitar riff like in the Stone's "Satisfaction", etc.
This is the hit making stuff right? Is that just listening to popular music or the music that you like and figuring out which bits of the song you like.. the old whistle test?
Again I guess what I'm asking are there objective rules?
The rules are, IMHO, to do what you think the song and the music calls for. Other than that, there are no rules.
Personally, I couldn't care less what everybody else does. It great to listen to a wide variety of music and productions, and to get ideas from others and other sources, sure. But that is different than purposely trying to follow some popular trend or "rules of fashion". The most successful artists are not the ones that try to copy others, but are those that follow their own muse and wind up wanting to be copied *by* others.
And what you hear in Cee Low's song is what YOU hear. What I hear may not be exactly the same thing. This is where things get fun; it's in listening to what the song "wants" and laying out our own interpretation on it.
It's like a movie; give a script to Ron Howard and the same script to Woody Allen, and you'll wind up with two different movies. Both will probably be very good, and both will serve the script perfectly fine, but they still will be two different movies. And that's a good thing.
G.