I've been following this thread from the beginning. Your case of finding the M3s "sibilant" is so bizarre and atypical that I've asked that your M3s be checked out in our lab when they arrive from our Michigan depot. They are expected today.
Alan
The results should be interesting...
SC-05, HLT6189, Oppo BDP-93, Blue Circle Audio FX2 Axiom M2v3's, QS8v3's