How to correctly set preamp and poweramp levels?

I have an Emotiva preamp connected to the poweramp section of a Harmon Kardon HK990.  When first connected, this combination produced badly distorted sound until I realized that the input sensitivity of the HK was mismatched to the output of the Emotiva.  When I turned the sensitivity down in the HK they sounded great.  But, how do you determine the IDEAL relationship between preamp and poweramp as to produce maximum efficiency for the combination?  In other words, how do I determine the ideal input sensitivity so as to have adequate poweramp headroom while still utilizing the amp's full potential? 
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.

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