Basic monitor setup
A mixer is dependent upon his monitoring setup, conditions and methods. If the monitors don't work in the environment, or the mixer (remixer) doesn't interact well with the monitors, then all other tips and tricks are meaningless.
When auditioning monitors, one thing that is frequently overlooked is how they are placed. Than can make a tremendous difference in the frequency balance and stereo field and should be dealt with before you involve yourself with critical listening. Before you settle on exact placement, consider the following:
Distance between the monitors
If the monitors are too close together, the stereo field will be smeared, with no clear spatial definition. Too far apart, the focal point or "sweet spot" will be too far behind you. You'll hear left and right, but not both together. Basically, the speakers should be as far apart as the distance from the listening position. If you are 4 feet from the monitors, then move them four feet apart so that you form an equilateral triangle. Who said you'd never use high school geometry? A tape measure will work fine. You can make fine adjustments in or out from there.
Check the Angle of the monitors
Improper angling will again cause smearing of the stereo field, usually resulting in a lack of instrument definition. To set the angle of the monitors, set them up in the equilateral triangle first. A great trick for getting excellent left/right imaging is to mount a mirror over each tweeter and adjust speakers so that your face is clearly seen in both mirrors at the same time when you are sitting in your mixing position.
The Position of the Tweeters
Most mixers prefer the tweeters of a two-way (bi-amped) or three-way (tri-amped) system to be on the outside (if you are placing the monitors on their side) thus widening the stereo field. Occasionally tweeters to the inside works but usually the result is once again, the smearing of the stereo image.
How are the monitors Mounted?
Monitors that are mounted directly on the meter bridge without any de-coupling are subject to comb-filtering effects, especially in the low end. That is, the sound travels through the console, through the floor and reaches your ears first (sound travels faster through denser materials) before the direct sound through the air, causing phase cancellation. One way to de-couple the monitors is with a pad of three-quarter inch open cell neoprene, but the best way to de-couple the monitors is with a set of Auralex MoPads which will not only solve the de-coupling problem instantly, but unlike a chunk of neoprene, (rubber) can be adjusted to angle the speakers downward directly towards your ears for optimal listening position (Relax, Mopads are not expensive.).
Instead of mounting the near-fields on the console, a better solution is to mount them on stands just directly behind the meter bridge. Not only will this improve the low frequency de-coupling, it will greatly decrease unwanted reflections off the console.
Check the Console
The angle of the console, type of materials used for panels, knobs, switches, the size and composition of the armrest, and even the type of paint will all make a difference in sound due to reflections causing phase cancellation. Another trick to see where sound is reflected off the console is once again to use a mirror: Move it along the console surface. Wherever you can see the center of the speaker cone in the mirror from your mix position is where the sound will be reflecting. Some mixers put a small piece of foam (such as those found in the Auralex ISO Producer Pack) on the console in those places when they mix. Of course, it's better to solve those problems with stands rather than to have bits of foam covering your controls, unless, you are mixing in someone else's studio - in which case, I recommend traveling with the aforementioned ISO Producer Pak to solve those problems on the fly.
If the sound of the near-fields on top of the meter bridge is unacceptable, then try moving them towards you with extenders or put them on stands behind the console. Don't forget to de-couple them.
