One simple way to remove sibilance is to select the sibilance only and EQ it out. The bad thing is that it affects the entire sound. However, if the sibilance is brief, the effect is not noticeable. You can always try to gate it too.
Izotope RX and many, many others make themselves out to be better than they actually are. You can always try Cedar as well but you really need to know how much and where to apply to the fix, otherwise you get nasty artifacts.
You can also do the "select the sibilance" thing semi-automated - combine a noise gate (or a similar way of finding hard attacks) with a bandpass to find where it is, then reject wideband passages (compare level in sibilance band with levels on the others), equalize only elsewhere.
I haven't found a fully automated effect doing that - when I did fix a thing or two like this, I used a Cool Edit macro. (it was called that before being bought by Adobe and renamed Audition) Probably could be done via scripting in Audacity too.
Sounds somewhat similar to a pop filter on everything - most instruments aren't affected.
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The distortion in this Decibully album is harmonic distortion in large amounts in all orders. Sounds like someone is overdriving microphones. Recording made with cheap mikes placed way too close to the instruments, vocalist also chews his one.
Alternatively, a very overdriven tube console.
