depends on the speaker. i would ask the manufacturer, but most would guess the tweeter at about ear level
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All of this is irrelative if the speakers are placed so they don't sound good to start with! Mainly, I'm talking about a "bass issue". You need to place the speakers where they're going to couple properly with the room/listening possition, for flattest most solid response, first and formost! Otherwise, you'll never get a good sound from the speaker! That's the foundation.
I find that, with 8' standard ceilings, 24" stands seem to be best, in general, for most smaller 2 way monitors. You get higher than that, and the bass get weeker, and there seems to be a hole around 63-80hz reigion. Anyway, try and see/hear for yourself before buying the stands.
If you have a 9' or 10' or better ceiling, you can most often get the speakers up higher, maybe 28-32" stands or whatever.
Still, if you end up with lower stands, you should aim the speakers up towards the listening possition for proper imaging and soundstage.
If it came down to a speaker manufactures(like Sonus Faber) recommending placing their speakers at a certain height for tweeter, whatever, vs. placing the speaker at a height that the speaker actually sounded good at, I'd go with the latter, every time!
Mapleshade Music has a number of suggestions, one I just read that I had never heard before. They suggested placing mini monitors on "their maple" platforms on the floor. They then tip the speaker back to point the tweeter at your ears. This does two things, first the relationship to the floor would deepen and tighten the base. Secondly this would help phasing of the driver and tweeter allowing for near field listening. This in turn will make the sound clearer and fuller.
I have no idea how this actually works, but it's definitely worth a shot. I would look at Mapleshade's web site and see if they have posted their suggestions. If they have not request a catalog. They have quite a few excellent tips on free tweaks.