Agreed to the above.
However, my thoughts are....
Bi amping does make a difference in the music sense. This will also make a difference on the cinema side also, as your main left and right speakers will handle the music sound tracks (Depending on the native soundtrack of the DVD disc...)
However, I would consider NOT bi amping, and spending better money, on a higher quality single wire cable.
E.G. QED Silver Anni Bi Wire, circa £10 per m. I would spend my money on a Chord Company Rumour 2, single wire, Circa £10-14 per m. (Plus plugs, etc, etc...)
Loosing your 6th channel, I dont think you will notice this. Baring in mind that not many DVDs or Blu Rays actually have a native 6.1 surround sound track! So all your getting is a bit of information from the rear left & right speakers, to 'fill in the gap' as the 6th channel.
My thoughts are, spend better money on your rear effect speakers, rather than comprimsing, and running 6.1 or 7.1.
HTH!
Ean.