I assume you have the ALC set at or close to 1200 watts. The ALC provides an extra layer of protection when driving a SS amp using a 200w rig. On my rig I also set it such that the max output is 100w no matter how far I crank the power knob. I don't know if the 990 has this feature.
With the cable in place, the ALC is handled by the rig, not the amp. The knob on the amp does nothing to the power output in this configuration that I can tell.
Your info about max power is well noted and the 990 does have that ability. I am still working out how I will use this feature. The thing is if the max power were 100w, it might be an easier thing to do, but if I m driving near 1200w at 45w power... then setting max to 45 seems less likely since running barefoot sometimes will be the case and 45w will just not do.
I have a question into Ameritron about the Max power and wills see what they recommend. For now, I am just being super vigilant.