The 975 result is similar to your earlier test when the MV was at -30. Here, you are able to play an "85 dB" test tone at 90 dB. That's theatrical ref level +5 dB.

The cable box is not defective because it is dutifully passing through the DD bitstream it receives. It cannot modify the levels in a bitstream.

Even though you normally use cable, do you have any OTA signals available? If you can get a network channel from a TV tuner (via S/PDIF) and compare the same channel from the STB, you can verify whether the STB is outputting the "normal" audio loudness or not.

Broadcast TV average audio is modulated much the same as other sources. If it seems to be 20 dB low on peaks, part of that may be due to TV programs having much lower peak-to-average range in the mix. But if you switch to a PPV or HBO movie, the full peaks should be present like on a DVD.

If, in the end, you really want more gain in the 975, give Outlaw a call and see if they have any ideas.