Your 990 has a maximum output level of 9 Volts. This is easily enough to drive the Adcom to full power. The Adcom distortion light comes on when the amplifier senses a difference in waveform between the input and output signals. This is usually caused by clipping but could be due to other problems in the amplifier. This amp is getting older and you may have some electrolytic capacitors that have dried out. The problem is not the 990.

Also, into reasonable speakers that amp will blast you out of the room at full power. It's got a fair amount of headroom over the rated 200 WPC into 8 ohms. I know, there's one sitting on a shelf here somewhere (in reserve for side surround if/when I get them)