the problem with using 7.1 with a 6 channel sound, you can make it 5.1
the reason is :
6.1 has : Left, Center, Right, Surround Left, Surround Center, surround right
7.1 channel has : left, center, right, surround left, surround center-left, surround center-left, surround right
there, you see the problem
2 center surround vs 1