Well , this is one of the most difficult issues , the midrange.
First of all , you mention that you mix for 10 years? i mean.. room balance & monitoring is one of the most important things , shouldn't that be fixed by now? i don't really know your situation as you might be mixing only for fun or whatever but generally from the starting point of anyones learning curve on mixing , there's a point - not very far - where you'll wonder why those things don't sound good enough , and you'll eventually stumble upon room treatment , so that's weird..
I'll try to emphasize more on the mixing techniques that can be some crispness in your midrange , i really can't get very detailed because there are thousands of things but ill try to cover some standard techniques that i follow , especially on hip-hop to mix the mids the way i like.
The following are products of my own research , meaning that i search about stuff that guide me to the sound i want to create so that might be far away from yours , but ill try to stay as more generic as i can.
The drums are allways the number 1 suspect in such situations , especially in hip-hop mixes so ill cover these first
You ask if there are some standard frequencies that should be accentuated/attenuated ,well , while i don't like giving such advise for the sake of this answer ill say yes.
I find that 8/10 of the snares i get need some attenuation in the 300-400hz region (you might even find people that will tell you to aim specifically for 315hz , but that's just too specific). This is to get some boxyness out.
Hi-hat: A lot of times , people use samples that are not treated at all , so you could end up with a hi-hat that sounds ok when you start , but muds the mix afterwards cause it plays all the time , definitely HP filter that!
Kick: For the same reason above , kicks tend to need some attenuation in the midrange especially in hip-hop , to create a nice blend of the attack and the bass. I tend to make a nice big dip in the mids when treating boxy kicks!
These are the first basic moves to get started.
More than EQing
One thing that many people seem to neglect , is duration.There's so much compression in mixing these days that the duration of the sample really starts to be a problem. While you year a nice punchy kick when you search through your samples , this kick might produce some wobbly/bassy tail if you compress it , or some Decay which is what the compressor left behind after the sustain.
This can really mud up a mix , and also mess very badly with the groove.What i end up doing in such situations is use a Gate. Choose how much duration(Release) i want and trash the rest , this helps clean up the space , and really give the kick some air to breathe down there.
Many times you'll find it easier treat a kick correctly by duplicating the channel and treating the bass/body part differently than the attack part.
So you can gate the bass/body part to create a fast and accurate bass explosion and treat the attack in an other way..
Action and Counteraction
Now that we EQed all the bad stuff out , we should accentuate the good stuff!
Snare : Go for the crack in the 2000hz region with a nice mellow Q and 1-3db+.This really makes a snare crisp!
Kick : find a nice snare - kick(attack) relation to the upper-mids and boost that so the kick sticks out too!
hi-hats : There's too much to cover about hi-hats , generally find a nice spot that the hihat sits well in the mix!
Presence helps things to pop!
The way i add presence (or one of the ways) is by taking a channel , make a duplicate of it (parallel) , emphasize the highs (2khz - 20khz) and distort that with a hi-end distortion box (or a plugin like Decapitator). Afterwards just mix it with the original till it "pops" (also you can EQ further after the distortion).
This applies to nearly everything in a mix, in its own way.
One of the best ways to emphasize the mids are the dynamics and the dynamic range of the mix.
Mid range is very complex and has so much stuff going on , if you compress too much , you'll create mud.
Let things breathe not compressing directly. Use parallel compression techniques and keep the dynamic range and also the control of the compressor.
Not much to say here, bass is one of the biggest problems for a lot of people for me it's down to 2 techniques that are much alike.
First of all treat your bass in 2 different parts , one that has the mid-highs and one that has the body/bassy part.
In the bassy part there are 2 ways to go.
- FET compression (google it) and LP Filtering
- Distortion and LP Filtering
What both of these will do is create a nice wide bass (in the bass region) that is very controlled and massive.
You can treat the other channel which contains the mids/hi-mids of the bass any way you like (even add Presence of this channel to a 3rd bass channel as i said before).
Guitars and other stuff
Use the panpot! Don't be afraid to go deep L-R as long as you keep some kind of tonal balance between the 2 speakers.
For example an acoustic guitar 70-30 could easily support a clean electric guitar (picked/strummed) 30-70 in terms of tonal balance!
Also go for separation , see up to which point you can HP(or LP) filter everything without altering it's sound, it might be a little but it really really helps!
Buss compression can really help things push around each other in a nice way. I like the SSL-Buss compressor (and the plugin too) i just apply it on my drum kit and compress to 3db , sometimes i add the bass too , it really depends on a lot of stuff , but give it a shot!
All these are mentioned with a very big YMMV sign. If you really have a problem with your mids ,start looking for separation , dynamic range , presence and other stuff like that on the Internet. You'll find tons of techniques and opinions.
Another big Tip i'd give to anyone that's mixing , is to always mix the whole mix. Don't try to approach things in Solo , you might like a guitar in solo , but it sounds rubbish in the mix . So don't mix solo tracks. Mix everything altogether , and let the song and the sounds guide you!
I don't know your standards ,but as far as my personal opinion goes when it comes to mids, don't blame your self too much. Digital technology sounds really really bad when in comes to mids.. This doesn't mean that it sounds like the problem you describe(which is a problematic mix), it just doesn't sound like a good Tape machine!
Hope that helps :)