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?