Question regarding codecs on Marantz NR-1501

[BIslander]

PLIIx intelligently steers some of the surround content in a 5.1 source to the rear channels, essentially taking sound that would phantom image behind the listener and anchoring it actual speakers.

DTS Neo:6 works much the same as PLIIx, except that Neo:6 produces a mono rear channel that is fed to both speakers while PLIIx produces stereo rears.

Bottom line: you can effectively expand 5.1 sources to 7.1 pretty much all of the time.