1) Is the HDMI issue the same for any of them (BD, STB), or is it maybe just from an HTPC? Does the power amp use a grounded power cord?
YES. IT DOES. I SWITCHED MY MC352 W/GROUNDED CORD TO ONE OF MY MC7100 NON-GROUNED CORDS - PROBLEM SOLVED.

2) When you calibrated the speakers, where did the gain trims come out? Are there any that are very low? Might seem silly, but could try shifting all of them up by 6 dB to see if that helps.
GAINS ARE CRANKED. STILL NOT VERY LOUD AT ALL AT "MAX" VOLUME. REALLY STRANGE - I'M THINKING IT HAS SOMETHING TO DO WITH THE INTERFACE OF THE 975 AND MY AT&T U-VERSE BOX. MY PIONEER SC-25 RECEIVER I WAS USING FOR PROCESSING DUTIES HAD NO ISSUES. BTW, I'M USING A OPTICAL AUDIO OUTPUT (I HAVE A PROJECTOR) FROM THE AT&T BOX TO INPUT #1 ON THE 975.