I had grungie-problems that turned out to be AC power related. I tried a lot of the things you've tried; finally figured out it wasn't the signal cables or the hardware. Look closer at your AC power line feed. You likely need some decent line-filtering & probably some upgrade AC cords too. Chang Lightspeed's line filtering products were my affordable solution to the grunge. Then I experimented with AC cords, & eventually installed a dedicated AC line for even further improvements. Don't mess around with your speakers until the input signal is clean first. They're probably fine as-is.
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.)