nice gentle slope from 20hz to 20khz in the spectral analyser on the master bus, with no big peaks or bulges in the graph, is how a good mixdown will look, load up your favorite track into your DAW and check out the spectral analysis and you will see straight away what I am talking about
but - having the right curve, doesn't necessarily mean that the mix sounds good, it can still sound muddy and messy if its not EQ'd right.
one of the biggest hurdles people face in mixdown, is not properly EQ'ing, any sounds which jump out of the mix, or disappear into the mix, with just tiny volume changes, are a good indicator that more careful EQ is required
dont worry about loudness or volume too much, concentrate on the overall balance
RE Kick levels, on the spectral analyzer (set to standard Log) you should see the sub rolling around slightly above the kick, and the snare just below the kick - check your favorite tracks for reference, rather than relying on numbers, coz numbers dont mean allot, its all relative