JZ,
Yow, both of those are pretty high off the ground!! Prepare to lose "bass reinforcement" with either of them. What's the reason for wanting to get them off the ground? To get the tweeters higher? To get better woofer/room interaction by getting them off the ground? To clear something otherwise "in the way?" At any rate, my experience shows that once you start getting woofers more than a foot above the floor, you
really start cutting back the woofer output due to much lesser room reinforcement. So, think about that a little bit.
I think I've got a couple of pairs of exactly what you're talking about, 16-inch height. I had my JBL 4312A monitors on them, and got another pair when I got my L100's so I could really compare the two systems. I got 'em that high mainly to clear the arm of my sofa, but I wound up later on putting the 4312A's on crates that were more like 12 inches high, and I preferred that lower position.
Stability-wise, yeah, these only have little "plates" to actually put your speakers on. Make sure to balance the weight of the speakers on them, and in the end they're pretty stable. Rough-housing kids and major earthquakes could pretty easily knock them over, but otherwise, there shouldn't be much of a problem.
Here's a picture of the two pairs of speakers on those stands, along with the big L150A's. Like I say, I'll bet these are exactly the kind of stands you're talking about:
I hope this helps -- good luck!

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