I used a 717 for a couple of years and then a 737 until a week or so ago.
Tried a 747 and a 757 and to me the differences were very subtle if any and not worth the change, I have a meridian processor so cannot use the sacd or dvd-a side of the 757, but thought the 747 was the better movie player anyway.
Oh and the 656 is not as good as any of them imho.
I did try a denon 3800 in my system too, and thought this better than all of them, but it would not play the first disc I put in it, Moulin Rouge R1, so it put me off a purchase, but if you are not having problems with it then I would say stick with it.
I decided that I would now just keep the 737 and put my money into D-VHS instead, but, then saw the Panasonic rp-82 in action, and ordered one the next day, awesome player with no chroma bug at all and the most natural colours I have seen other than the tag Mclaren. One thing though, the audio side isnt quite as sharp as the pioneer, for music I prefer this but on films it was quite nice to have it a little sharper.
What display device are you using ???
The progressive scan is also excellent, I am now using it instead of my lumagen, but it is region 1 only, for the 2% of pal discs this doesnt bother me though.
Isn't Buns selling his RP-82 for £300 in the classifieds at the moment ??