End of a long quest to vanquish sibilance.

Is it possible then that since it's not height, it could be the increased rigidity of the speaker vibrating the tweeters? 

@philliprcook 

If the stands themselves had been shortened, then I suppose they would be more rigid. But that's not the case, here.  

Now the stands are in contact with the hardwood floor, which certainly has more mass than the pavers, if that matters. Also, I wasn't using single pavers -- I had two pavers butted up against one-another under each stand. Perhaps that was a factor? 

Perhaps someone with more expertise in such matters will comment.

Concrete, being a denser and stiffer material than wood, tends to have a higher resonant frequency and may be a reason for increased high end.

@philliprcook 

You may be right. From what I can tell from listening, it seems to be more a function of the material than the height change. 

There are two sources of sibilance, recording generated and system generated. There are a few recordings that are sibiant no matter what you do other than generate a Grundy Notch Filter, will remain sibilant.

System generated sibilance is virtually a universal problem. Everyone is aware that the easiest way to cure sibilance is to turn down the volume. Everyone should be aware that the frequency response of our ears changes with volume. Fletcher and Munson demonstrated this and generated a series of frequency response curves or Loudness curves. As volume increases our ears become more sensitive to Bass and treble. If I remember the inflection point is at about 3500 Hz. To achieve the same loudness at say 90 dB that you have at 70 dB you have to decrease the bass and treble about 10 dB. A flat curve never sounds right. It will have to little bass and too much treble. If a system is tune to sound right at 70 dB when you turn it up to 95 dB you will have way too much treble and decent bass. The end result is sound that makes one squint. To add insult to injury our hearing is most sensitive at 3500 Hz and it is in this area that sibilance is most prevalent. This is also where the BBC's Grundy notch filter or curve is active. There are only two cures for a sibilant system, use a lower volume that is not sibilant and digital signal processing.  There is a volume for each individual recording that sounds right depending on the volume it was mixed at. You either play at that volume or generate an equal loudness curve for a higher volume. I have three different equal loudness curves programmed, one of the three usually works well. For bad recordings I have a BBC Grundy notch filter I can add. 

Fiddling with equipment is a costly and usually ineffective way of dealing with sibiance. Sound absorption at all the first reflection points might help, but won't get rid of the problem entirely. The best way to deal with this was the dynamic loudness control in the Tact 2.2X and it was wonderful. The system sounded exactly the same regardless of volume. Unfortunately that unit died after 20 some odd years. So, I bounce back and forth between the three curves and listen at the volume things sound right.