AFAIK, the differences between the PX60 and PX600 models are that the 600B has more features on top of the 60 - e.g. better sound system, Picture-in-Picture, SD card slot for recording/photos.
However, there is no difference in picture quality (they both use the same 1024x768 display) - in fact I verified this quite rigorously at John Lewis yesterday
They had a 42PX60 and 600 setup side by side, both feeding off a Blu-Ray drive playing Fantastic Four in HD. At first it seemed like the 600 really did have a better picture (the 60 was showing aliasing around fine edges), BUT this turned out to be due to the 60 being fed via Component, while the 600 was being fed by HDMI. One of the JL assistants swapped them round and the 60 matched the 600's HDMI quality like for like, while the 600 went down in quality when put through Component - as I expected
These were obviously the 42" models, not 50", but I expect the differences between the 50PX60 and 600 are exactly the same as between the 42" models.
As for the PH9, I'm not sure - but isn't this simply the business model which comes without any speakers at all (i.e. like a computer monitor)?