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According to Stereophile, the HK990 has a line input sensitivity of 350mV and an input impedance of 43K ohms. The 350mV sensitivity is much lower than most any other amp I've ever read/seen which usually implies a very narrow control on the gain. The 43K is fairly normal for solid state amps. I don't however see anything indicating either of these two parameters are user controllable. How are you adjusting the sensitivity?
On the front panel of the HK990 there is a button labeled "Input Setup". By pressing this button and scrolling through the various inputs, the Controller input is listed. After selecting that option the adjustment is made on a decibal basis ranging from -24 db up to +12 db. I'm assuming that changing these values results in a change in the processor input sensitivity because the system volume changes even though the preamp volume is unchanged.