From what I've heard from the product manager at Korg USA, the PA1XPRO still adds many features not found on the PA800. First, there are sliders for all the parts and for the draw bar organ footings. Next, you can have expansion boards installed into the PA1X which cannot be put into the PA800. The vocal harmonizer in the PA1X has more options such as vocal voice modeling which allows you to download new features (at a cost) if you want these. And the big difference is that the PA1XPRO still has the nicer feeling keys which are both 76 in number and more like playing a piano in length and feel, espcially the black keys, which on the 61 key models are shinny and shorter and a longer, flat finish on the 76 note model.
According to Korg, the sound engine is the same on both models.
George Kaye
Kaye's Music Scene
Reseda, California
www.kayesmusicscene.com