That's an interesting experiment with the fronts and the center pre-out connection.
As you said, the distortion was still there so perhaps it is a source problem then.

Was the sound originating with any type of frequency or just sounds in a certain range?

The Diana Krall music was helpful for determining the frequency range causing any problems because of her low and high voice accompanied by the cello notes and piano notes. Holly Cole Trio has very similar music.

"Those who preach the myths of audio are ignorant of truth."