Monitor placement height

General rule is that the tweeter be at ear level.

If they are near fields they should be at ear level. A near field has a very narrow dispersion field. They should be pointed to the point where your head is. The ideal is a triangle where the near fields position form the base line and your head is at the apex. 

sharke
What about angled up slightly? My isolation pads do that but I have little idea why. I figure it must be a plus otherwise they wouldn't have made them with a slope. 

Again because near fields have a narrow dispersion of high frequencies as long as you are close to the axis of the tweeter not off axis it shouldn't matter if they are tilted up a little. You may need to adjust them with a wedge that will allow you to have them pointing at your ears.  In contrast normal loud speakers try to have as wide of a dispersion as possible. But keep in mind that normal speakers are not supposed to be listened to too closely. They are not meant to be toed in either. Think of near fields as more like headphones. Not that they are. 
 
One point though they shouldn't be so close to you that you can discern a sonic separation between the woofer and tweeter.  

John
If they are near fields they should be at ear level. A near field has a very narrow dispersion field. They should be pointed to the point where your head is. The ideal is a triangle where the near fields position form the base line and your head is at the apex. 

There's probably something more ideal than this ideal.
 
Let us put things into perspective here; Any three points form a triangle and the three apexes are are called apexes.
 
If you define a line between two points as a baseline, a third point placed anywhere in Cartesian space that isn't on that line (and its linear projection) will form a triangle.
 
Placing the speakers and the listening position (a.k.a. your ears) in the formation of an Isosceles triangle is traditionally thought of as nearly ideal.
 
Some people regard an Equilateral triangle ( a subset of the Isosceles set) as an ideal.
 
As an example of an equilateral triangle layout; If the line formed by the speakers is 3 feet from driver to driver than the listening position will be 2.598 feet measured perpendicularly from the center of that 3 foot "line".
 
I don't have strong opinion about what sort of Isosceles triangle that I use. I tend to gravitate to something close to an Equilateral choice or sometimes a layout where the vertex angle is smaller than the base angles. If the vertex angle is much wider than the base angles the stereo spread can become unnaturally wide. I think that consideration of proximity to side walls should be incorporated into the decision with a desire to keep as much distance between the speakers and the side walls as practical or possible. In many cases a regard for proximity to the side walls will preclude a placement where the vertex angle at the listening position will be wider than 60*. In the end it just depends on where the listener places their head. If you start with an Isosceles triangle layout you can move forward and hear the stereo "spread" wider or move backward and hear the stereo "blend".
 
 
 
Turn it up!!!
 
 

In my experience, the tweeter and woofer need to be equidistant from your ears. The monitors should also be vertically oriented so that the two drivers are panned no further left or right from the other driver(s), and the distance apart is critical to the speaker's design. Use the manufacturer's recommendations, if they offer suggestions.
 
Angled speakers on pads, I believe were assuming the speakers are situated slightly above your ears on top of a flat hutch or surface above the mixing console.
 

brconflict
...the tweeter and woofer need to be equidistant from your ears... 

Yes, and the most important thing is that the tweeter and the woofer are "in phase".
 
Some designs match the physical placement of the voice coils, other designs make adjustments in the crossovers, and many designs ignore the issue altogether.
 
I admire the designs where the mid point of each driver's voice coil is time aligned by physical placement. If this is the case the "front" of each speaker may or may not end up equidistant from the listener... but the mid point of the voice coils will be equidistant to the listener.
 
 
For example; This is an illustration of a time aligned 3 way system:

I use small JBL monitors (LSR2325) and have them mounted on the wall above and behind my LCDs, but aimed at the middle of where my head is when I'm mixing. Works fine...the tweeters are probably 18" above my head. With two 23" monitors, you don't have a lot of choice that doesn't put them too far apart.