End of a long quest to vanquish sibilance.

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.  

@mijostyn 

All I can say is that I haven't experienced any noticeable link between volume and  sibilance at the levels I typically employ. Doesn't mean it's not happening-- I simply don't hear it. Sibilance might get worse at levels higher than 75dB but I never push the volume that far. Below say 65dB, my system doesn't come alive. This could be due to the size of the space relative to the small drivers and/or noise floor of the components. 

Happily I only had trouble with sibilants only with one speaker solid state preamp decades ago. Never since at any volume.

@stuartk Many people habitually do not play their systems loud because they do not like it or they live in a situation where it would be unacceptable like an apartment complex. I promise you if it sounds balanced at low levels at some point when you turn it up it will make you squint. This assumes that you are not a Martian or deaf. 

@ghdprentice Neither do I because my system is tuned for high levels. Unless you can accommodate, your system sounds dull and bassless at very low levels. This assumes you are not a Martian or deaf.   If you are telling me your system sounds great at 95 dB and at 70 dB without a frequency response adjustment you are a Martian and deaf to boot. What human ears do a various volumes is a scientific fact. 

@mijostyn

Now that you mention it, on occasion, when I’ve cranked the volume so I can hear it better in another room, I’ve noticed a fatiguing edge. I never turn it up that loud when in my listening chair, though.