Many receivers will use NEO:6 to create a 6th rear speaker from a 5.1 DTS encoded source. Others, such as PLIIx, will create 7.1 from a 5.1 source.
Curious though, when you say 5.1 preouts, are you using the analog outs? And if so, why? Unless your DVD player has superior DACs than your receiver, I'd use a digital (optical or coax) cable.
Jason