I'd like to chime in here if I may? Guitars (especially dirty guitars where there are several) depend on the tone as much as how well/consistent they are played. The biggest issues to watch for other than playing that is not tight or consistent:
Too much gain: This will totally destroy several guitar tracks that are playing the same part. If you do any type of chugging, it will sound like a run on sentence that never breathes or accentuates any accents.
Too much low end: This will mask with your bass guitar. Don't be afraid to high pass. You may end up high passing all the way up to 250 Hz if your tone has lots of low end in it. This low end will even make a "whoomfing" sound which will raise your eq level in certain areas. You will see level increases when you chug a Ab, A, Bb and B more so than any other chords. These will ramp up your LED. When you use excessive low end, these chords can make your tone go off the charts with bass.
To listen for this low end, high pass until you no longer hear a low end air push behind your tone. You'll know when it's gone....the tone will leap out at you in a good way. Most guitars do not need any low end at 80 Hz or under. But this depends on what is in your tone or not in your tone. Sometimes a few harmonics of a low frequency can enhance a tone just a bit....so we don't want to totally kill the thickness of the tone. But we do not want it to have much low end beef in it that could walk on the bass guitar because this will also interact and mess up any other guitars you have within the mix.
Too much effects: This will bury your tone and make it appear behind your backing tracks and can even mush things up. If you are a sloppy player and hide behind effects, this will make your tone even worse and will make it harder to blend. Don't use long reverb tails other than for a special effect in a spot or for something artistic. Long tails will wash you right out. Watch for long delays because if you have a tone that has the above issues I've already mentioned, they will repeat so then you have double mush.
Watch for chorus effects. Though they make you sound thicker, the low sweep of the chorus can add bass and artifacts that wash you out or mask you with something else.
Mid range: Too much mid range can cause mid range congestion within a guitar tone. It will sound honky and will mask in with piano/keyboards and even lead vocal or anything that lands in a mid range type frequency field. We want some good mid range thickness and girth, but we also want the tone to be heard and have an identity. The frequencies to watch for here can be from 200 Hz all the way up to 860 Hz. So experiment and try your best to stay out of the ranges under 400 in excess.
High end: Sometimes guys mistake excessive treble for quality distortion. Meaning, they have a razor sharp tone that is cutting way too much. Add enough guitars with this type of sound, and it's going to just sound bad. The problem with high end is....it is going to be a bit more subjective than the other guitar frequencies because sometimes a tone that cuts through the mix for rhythm is the right choice where one with less high end cut may be a little too warm sounding. I like a little sizzle in my tones but not enough to where it pierces the ears or sounds annoying. Finding this happy medium usually depends on the material you are playing on. If you find you are a little razor sharp and high endy, try low passing until the sharpness is less apparent.
Like sounding guitars: If you have too many of the same guitar sound, you are just boosting volume. The same if you are panning several guitars the exact same way. If I were to use 4 guitars, I'd pan two wider that are my main guitars and would bring in the other 2 on a chorus to push it harder using a tighter pan with a little more high end push.
OR....do it the other way around and pan the main guitars tighter and bring in the other two out wide for a chorus with a different tone. I like my pans to be a little tighter than hard left/right. The reason being, I tend to personally have tighter sounding mixes when using tighter pans. This also leaves my lard left/right for special items I want to leap out to my listeners. There are no rules here, the above is just what works for me.
Effects panning and eq: This to me is pretty big because not many novices know to even do this. Every time you use an effect that isn't a mono effect, the effect is being placed in stereo which is equal to hard left/hard right. Do this with enough effects and you can wash out everything. Though some use a LCR method, if you are not careful that method can really mess you up and I'll tell you why.
Too many effects hard left/right are like a bunch of tracks hard left/right. This is all well and good if you know how to treat stuff like that. BUT.....we don't want all of our effects to reach all the way out there.....AND we need to eq them so they don't wash out our sounds. I'll explain it a little deeper.
For example, say we have a lead guitar up the middle. We put a stereo ping pong delay on this guitar which spreads 100L/100R. In the 100L/R pan field, we already have two rhythm guitars. If we allow this delay, it is going to walk right on top of our rhythm guitars. If you eq the delay, you *may* be able to keep it in the 100L/R pan field. But most won't do that and will wonder:
a) why can't I hear the delay that good?
b) why are my rhythm guitars a bit less dominant?
So to stop this from happening, you keep your effects within the context of your instruments and you will keep things nice and tight. Along with eq-ing the effects, you allow them to sit in the mix better with your effects. Here's another example...
Say we have a reverb on a snare drum. The snare is centered, but the stereo reverb is spreading all the way out 100L/R. You are either going to:
a) hear a verb all over your mix that disconnects your snare from your kit
b) will not hear enough of the verb to give the enhancement you are looking for
To control spread of your effects, use the Sonitus Phase plug to control how wide your effects go and eq the effects either using pro-channel on your busses or drop a Sonitus eq right after your effects and clean them up.
*High pass reverbs to remove the low end from them if need be
*Low pass reverbs to stop them from hissing
Do the same with chorus or flanger effects. Try to make delays alter from the original tone if the delay default seems to get lost in the mix. High passing it usually cleans this right up.
And finally, high gain or rock guitar tones need to be recorded just right in order to work effectively. If you don't have any clue what a good rock or metal tone is supposed to sound like by itself, you are going to have a hard time with this. The reasons being....
Most guys think they have created the ultimate guitar tone when they play through an amp or VST. The problem is, the sound is all alone. An all alone sound that blows your socks off will most likely NOT work in a mix with other instruments. The reasons being....all that I have mentioned above as well as knowing what a good guitar tone sounds like. We can name lots of guys we love tone wise...but when you yourself cannot achieve those tones, this field can become VERY challenging.
All alone tones vs. tones that work in a recording are two different animals. There will even be times where you may not like the sound of a guitar when you solo it up in a mix. However, when you bring it in with all the other instruments, it sounds great! Finding that happy medium is important, but you have to remember...
You have automation. If a sound is all by itself in the mix, by all means pull out all the stops and make it sound the way you want! There is nothing else competing with it...so blast away the way you hear it best! But, when you mix and have to make the guitar sound good with the other instruments, there are times when you may have to sacrifice what the tone sounds like all by itself. Remember that THAT doesn't matter because the tone is NOT by itself. speaking of that....one last point.
Eq-ing solo: I feel quite a few people fail at mixing because they solo up tracks and try to make them work. They spend hours and hours trying to get tones from instruments that sound great by themselves all alone, then they wonder why they sound terrible when they include the instruments in the mix. As a teacher in the recording field, I can't tell you how many times I see people doing this to the point of frustration. When you are well seasoned and know what to listen for, solo is your friend for basic starting points within a new mix as well as pinpointing problem areas. But to use solo exclusively to eq.....this can be a downfall and a time waster unless you know what you are doing.
The more you mix and learn to identify sounds/tones, the easier this is and you just know what is right and what isn't. Until then, try your best to mix within the mix trying the above methods. This should help you improve ten-fold. If it does not, you can always post up a clip of the tones being used and we can rip them apart and tell you how to fix them or whether you should trash the tone and try to recreate. I hope some of this helps you guys. As always, best of luck!
-Danny