Muzz,
You shouldn't have any problems, since the AVR preamp section will still act as a preamp/controller for the main left and right front speakers, feeding the preamp signal to the main-in jacks on the hk990. And the master volume control on the AVR will still function using the speaker outputs from the AVR for the other channels.
You'll be able to individually adjust the levels of each channel to compensate for any level differences the same as you would if you were using all of the AVR's internal amplifiers. The level adjustments take place in the preamp section.
Regards,
Alan
