Hi Francesco,
good question. :-)
As far as I can say after mixing music for almost 35 years now (27 years thereof as part of my main profession as a sound engineer), it's most likely a mixture of composition and orchestration, and even more so a question of your programming and mixing habits. Monitoring might be a part of the equation, too, but if you are used to the acoustics of the environment you work in, your brain should be able to "linearize" your hearing experience at least to some extent.
I often see people using the same approaches to a problem over and over again, just because they are reportedly "tried and tested", quite often suggested by other folks who used them _once_ with success, in a completely different context. The main secret behind good mixes is that there are no recipes, no "best solutions", because you will have to re-invent the wheel a bit each time, for each piece of music you're working on. Like a good arrangement, a good mix with nice and clean (but warm and well-defined) low mids will rely on a healthy combination of standard solutions for most tracks and a few special ideas you have to come up with for those unique ingredients.
Keep in mind that a good mix is not only about proper frequency distribution, but also about the envelopes (i.e. the changes during time) of each frequency band. For example: In most cases, having loud and "fat" bass is great, as long as there is enough dynamic variety to keep some "pulse" going. Another example: We like sparkling hi-end for the occasional cymbal hit, but continous energy in the top-most octave will annoy our ears pretty fast.
The basic question of a good mix is: "... louder than _what_?" This means: Every mix is some kind of hermetic little universe with its own, unique interrelations. Something has to be soft to make something else appear loud, something needs to be bright to make something else sound dark. Some signal has to be "wet" and far away to make another signal seemingly dry and close. - The balance of all components is what counts (but it might be completely different when mixing the next piece).
Coming back to your initial question, the first step for you could be: "If there are to many low mids in my mixes - what balance changes do I need to make them sit properly?" More bass? More high mids, or more treble? Do certain instruments need more space, or less reverb? Are there too many voices occupying the same range, be it with their fundamental notes or with their harmonics ...? As soon as you have identified the most obvious source for missing contrast, you can find a solution for that. Proceed from there! :-)
Sidenote: It's also good practice to work with a few reference tracks. They don't need to be "similar", necessarily, just "good sounding" in a general sense of the word. I like to call reference tracks "the north pole", and I use them as my compass for guidance through the haze of technical possibilities and the hidden persuasion of sound. ;-D
Mixing is a bit like walking along a spiral: You will work on similar tasks again and again (volume balance, stereo balance, frequency balance, depth, special FX - volume balance, stereo balance .... and so on). The main progress happens when each time you start over, the necessary changes will get smaller and smaller. - The decisive part is to realize when you don't walk along that spiral any more, but you're already in the center and actually running on the spot: Then you're done! :-)
(... sorry if this answer got longer than expected - I got carried away.)
