I was wondering about that myself. The Radio Shack guy told me to use the A and Slow settings. And of course everything sounded fine until the subwoofer which even though had the neighbors peering out windows didn't register until I switched to the C setting.

Since I spend a lot of time manually adjusting levels on the sub anyway between movies and music I didn't worry about it. And the improvements were major. But I would also like to know whether to use A or C or both for best results.