08-17-08: GrinnellThis review MENTIONS both the ZPhono and the 640P, and of course they're on the same list because one is $149 and the other is $169, but the review makes it very clear that the ZPhono is not in the same league as the 640P. At all.
I dont think you will see much of an improvement. I have the PL2 and had the phono board and got a Parasound Zphono with a TT and did not see much of a change. The Cambridge gets mentioned in the same list with the Zphono.
I have several Parasound Z components and a 640P. The 640P definitely operates at a higher level. As soon as I took the 640P out of the box it put out a contract on my phono noise floor. By the time I plugged the 640P in, the noise floor was gone.