I've tried the PRS+ image before and it gives me a warning that my firmware version is 1.1.something and it requires 1.0.something and that it will have to flash my firmware to the correct version. It then get about half way through doing something then just freezes and wrecks the firmware. I have to use the Hack 300 to fix the firmware back to the way it was.
I have tried a different original firmware from the http://projects.mobileread.com/reader/users/porkupan/
folder and it had the same MD5 error. I have been totally careful with the downloading of the firmware and check the MD5 on the computer. I even once generated a new MD5 file using the checking program and tried that and still an MD5 error.
I am out of ideas too, it's so weird!