Re: lower midrange vs. upper midrange frequencies
20-40hz = super low bass (not much there in most music and most speakers can't reproduce it very well.)
No the rest is a bit subjective but this is how I visualize the remaining 9 octaves.
40-80hz = low bass
80-160hz = bass
160-320hz = upper bass
320-640hz = low mids
640-1280 = mids
1280-2560 = upper mids
2560-5120 = low treble
5120-10,240 = treble
10,240-20,000+ = upper treble
If you really want to get a handle on each range of frequencies and how energy at those frequencies effects the music try playing some of your favorite music through a 15 or 31 band eq or a parametric eq and start fiddling with the knobs. You'll soon get a handle on what a frequency range "sounds" like. Lots of mixing consoles have a semi parametric eq for a mid range control that will let you fool with boosts and cuts from 100hz to 5000hz. The meat of music is in that range. Also, record your guitar and play it back and fool with the eq. There's no right or wrong setting to shoot for at this point. Just listen to how the sound changes for better or worse.