Take the left and right line outs of the 990 and plug them into a line input of your stereo preamp. When listening to movies select this input and when listening to music only, select whatever stereo source you want. For calibrating the system, set the stereo preamp volume to a repeatable position such as 2:00 o'clock and then do a normal calibration on the 990. When listening to movies always return the stereo preamp's volume control to this reference position. For music listening you can change it to whatever you want of course.

This type of setup completely bypasses the 990 when listening to music only.