Talk about a subject with a BUNCH of variables. Much of this will have to do with the configuration of the drivers as mounted on the baffle, the shape / angle of the baffle, your personal height and your seated listening height.
If using a "flat faced" speaker with a tweeter mounted at the top, the tweeter should be just slightly above ear level. This has do do with the fact that sound "falls off" as it travels away from the speaker. Think of the trajectory that an arrow takes if you were to launch it parallel to the horizon. It initially flys straight out and then begins to "nose-dive" towards the ground. The sound from most conventional speakers do the same thing.
If using a speaker that uses a "D'Appolito" or "MTM" arrangement ( most are on flat baffles ), the tweeter should be very near ear level. This driver configuration tends to project sound out very evenly in the horizontal plain but has limited vertical "spray". That is why it is called a "controlled dispersion" design.
If using a speaker with an angled baffle, the tweeter should be positioned slightly below the ear when seated. How far below will have to deal with the slope of the baffle and the seated distance from the speaker. Obviously, some experimentation would be required with this type of installation.
One needs to keep in mind that the height of the speaker above ground will also alter low frequency response. This has to do with "floor bounce", reinforcement and room nodes. Since the tweeters are now firing right at or very near your ears, highs tend to be accentuated. At the same time, you've lost the reinforcement that one normally obtains from having the low frequency driver(s) near the floor. The overall sound can be somewhat "lean" with optimum tweeter placement. That is, unless the designer actually knew what they were doing.
Since many products are "under-engineered" or "built to a price point", you may need to "trick" the speaker into thinking it is closer to the floor than it really is. One can do this by what we call "beards" or "baffle extensions". In its simplest form, a "beard" is simply a board that is attached to the front of the speaker or stand below the bottom of the cabinet. By changing the length and angle of the beard, you can effectively alter the amount of low frequency reinforcement and dispersion characteristics. While the end result might look "unconventional", someone that is both "handy" and "creative" can make this look at least "presentable". Building stands that incorporate a "beard" into them and then finishing them to match the speakers can really make things look nice and improve performance at the same time. Sean
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