I use the same setup as buckeye and it works fine. I think ewiler is referring to when you use both balanced and unbalanced to the same amp channel and would be the case if you are running both balanced and unbalanced to the sub.

There is always debate between the use of balanced or unbalanced and the benefits when the runs are short. Would the switch at each channel on the back of the 7500/7700 allow a direct comparison between the two types of cables? Or won't the test work because of the connection of both types of cables at the 990 dictates performance as ewiler has said? If the test were valid you would need to take into account the 6dB gain when using balanced connections.