Sibilance # 2 or "Boy, do I feel like an idiot"

I doubt it is silver…

Anybody want to weigh in on @holmz ’s theory that it’s IMD?????

My theory is that if it is IMD, you are going to need an O-scope, and not just handfuls of cables to try willy-nilly. That is just an exercise in futility, unless you have some reason to suspect that a super high capacitance is causing an oscillation. We would not know that without knowing the amp, speakers, and speaker cables.
But we do know that an silver cable (etc.) before the preamp and headphones, are the cables that you are looking for.
So entering into “cable-boccacci” is throwing solutions at a problem, that we do not know can be fixed with a cable. Why not pyramids, fuses, cable lifters, etc. as well?

If it was hearing, then it is possible that it could be the muscle that pulls the drum tighter to make loud sounds less loud.
It is possible to have that autonomic system become non-operational with extended ‘super concentrated’ listening. It is not overly common, and if the mrs is hearing it from the kitchen, then we can 99% discount that as a reason… Just not 100% discount it.

I have a system build around a pair of Lowther DX3, and being a DIY enthusiast I build my amp  (el84 SE) and back loaded horn for the DX3. I've also experimented a lot with cables witch I DIY from bare wire in natural fiber sleeves. 

To make a longs story short, I had sibilance issues with the DX3 that I mostly solve from changing the conductors in my DIY cables from 24 awg OCC copper to 20 awg OCC copper. The larger conductors shifted the tonal response of my system towards the low end reducing sibilance.

Sorry to hear about your issues, extremely frustrating!  After reading through both posts I echo the thoughts of sns.  Perhaps something has changed in your speakers.  This is the only part of the chain that has not been swapped out right?  If they are around 17 yrs old it could be a crossover component aging.  Can you somehow verify the sibilance is coming out of both speakers?  How about opening them up to put eyes on the caps, etc?  Maybe your buddy is knowledgeable about this stuff?

The dirty power scenario is interesting also.  Are there any new appliances in the house?  Any construction or changes recently?      

@pkatsuleas 

I followed sns's instructions, put ear close to tweeters but could detect no difference in presentation between the two-- the sound as a whole and the sibilance-- sounded identical. I will ask a friend who is more experienced than I am to check caps. 

No new appliances but I plan on trying a power conditioner just to see if it will make a difference. 

@felixa 

Thanks for your suggestion.