"What is the ideal height of near field monitors?"

There's no singular answer to this question because it depends on the configuration of the listening environment, but in general you should have them set up so your ears fall somewhere between the height of the center of the woofer and the tweeter. The more "on axis" (see WFTD off-axis) you can be the more accurate the response of the speaker system is likely to be.

There are, however, some factors that may modify what is ideal. The most notable of these is the placement of a large surface between the speakers and your ears. This could obviously come in the form of a mixing board surface or just the surface of a workstation desk. Unfortunately whatever you place between yourself and the speakers is going to cause some type of problem due to reflected sound arriving a few milliseconds out of phase at your ears. It's worth taking some steps to minimize this. Given that you can't cover the surface with acoustic treatment, the only practical alternative is to experiment with the positioning. Sometimes changing the angle of the surface can make a big improvement, and sometimes changing the height and/or angle of the speakers will help. Try raising or lowering them a bit, and try some different angles to see if you can find a sweet spot where you get the best response.

Some users place speakers on their sides. Many times this is done to clean up sight lines over the console's meter bridge. In general we recommend you avoid it. If speakers sounded better with the drivers positioned side by side they would be built that way. The problem with this configuration is that when you move your head side to side you change the relative distance of the tweeter and woofer to each of your ears. This causes a smearing of the frequencies around the crossover point, as they are being produced by both drivers. When drivers are positioned so one is on top of the other, side to side movement has much less effect (especially if they are at the ideal height), though up and down movement will have more effect.