What you will/may notice when a tube amp is driving a speaker with low impedance dips is an uneven frequency response, e.g. the speakers may sound "bright".I read this comment again and realize I probably ignored the the second part of the comment about how this could make the speakers sound bright.
This got me to thinking and I re-examined my setup one more time. To make a long story short, I finally realized that while I was moving equipment in and out, I had moved my Synergistic Research interconnects from their customary location between the DAC and the the Preamp. I moved them back and the sibilance is greatly reduced.
Thanks for listening,
Dsper