A sibilant problem is often a problem with the speaker. You want warm, laidback without losing detail? I suggest you listen to a pair of Vandersteen 2ce Signature speakers. I really am not familiar with the Mordant Short speakers you use but there is no sibilant attenuation with Vandy's. They sound like what you may be looking for. And they work very well with a wide assortment of electronics. Good luck.
smoothing those sibilants...
I appreciate the feedback on the interconnect post I made a few days ago, and here is my next question: If I am attempting to smooth out the sound of a "low-end-of-the-high-end" system, reducing grain while retaining detail (and looking for a warmer more "classic tube sound"), where is my energy/$$$ best spent? Would it be the digital source itself, the interconnects, the input tubes, the speaker cables, or the speakers? Or something else? (System info can be seen under the post entitled "need interconnect advice", and there are some new interconnects on the way.)