This has been moved to the HT area as it's more hardware related than it is software related.
Josh, I've only tried a model or two from Panny (don't recall which ones exactly) when I was messing around with a DVD-R. But they played them just fine.
I see no reason why your RP-91 shouldn't play a DVD-R. Especially as Panny has been selling DVD burners for quite some time now.