Doug:

Actually what I did is calibrate the first sub with the second one off to 75dB. Then I turned the first one off and the second one on and calibrated it to 75dB. Then when I turned them both on, they should be approximately +6dB higher (twice as loud) or 81dB with both of them on. That means you're getting the correct benefit of the 2nd sub. You can then drop the Channel Calibration - Subwoofer down to calibrate the dual subs to your final desired SPL. I like mine +6dB hot because I've got a big, wierdly shaped main room. So now I'm calibrated.

The problem lies with the fact that the "Channel Calibrate - Subwoofer" on the 990 works for PL II but NOT for Stereo or Upsample. So if you leave the Channel Calibrate - Subwoofer at 0dB and adjust your sub volume, everything is fine.

The minute you adjust the Channel Calibrate - Subwoofer away from 0dB, the SPL of the subwoofer tone is now different between a PL II mode input and a Stereo/Upsample input. This is all with the 2CH Sub Offset at 0dB. If you had to drop the Subwoofer offset to -5dB for example to get it to be 75dB, when you switch to a Stereo or Upsample input, the SPL of that sub will be 80dB not 75dB.

That doesn't seem right to me.