If you have calibrated your 990, the 0dB setting corresponds to a peak sound pressure level (SPL) of 105 dBa at your listening position from any channel with the program material at maximum volume.
The effects channel is offset by 10 dB upward, giving a max SPL of 115 dB.

The power used to attain this sound level can vary wildly depending on room size and speaker efficiency. On the upper end, Klipshorns give about 104 dB with 1 watt input, so you'd be at just over 1 watt power. On the other end of the scale, some of the old AR (Acoustic Research) speakers were at about 82 dB at one watt. This means 23 db increase in power for 105 dB output; that's about 200x, so there's your 200 watts.

Since music is so dynamic, it's very difficult to get a good reading of the output power level. Just to make it a little more interesting, speakers are reactive, so you can't just measure voltage and current and multiply to get real power.

Someone probably sells some kind of display that will show voltage levels either tapping the preamp outputs or power amp outputs, but I've not searched for one. The amplifier power can be calculated approximately using the voltage and speaker impedance with the old V^2/R formula,
If you are using the preamp output, you can use the gain of the amp to calculate the voltage applied to the speakers and work the math from there.

Steve