Actually when you do the math, it comes out to 6dB being twice or half as loud. It does require 10x the amount of power though. gonk is also correct that 3dB is double the power.

If that doesn't make sense, just remember that this is all logarithmic. It is based on curves. Each additional dB is more than the previous. Here is an article with way more info than you may have ever wanted.

But either way, even if calibrated differently, the two systems could be correlated volume-wise if we know what the two calibrations used are. If I set up with 0 on the display set to 60 dB speaker output and someone else set 0 to 80 dB output, then setting their system to -20 dB on the display should reasonably match my system's volume. What is really pertinent about this is that this will be the case if the 990's display is measured via dB and not some other arbitrary system. As long as we are comparing dB to dB adding and subtracting reasonably works.