Do you have a chance to do any critical listening after you install the new rectifier? BTW, which rectifier did you get?
Sorry for the delay in reply. I have tried 2 aftermarket rectifiers. Both are 5U4G type, and both fit in the case's top hole (with clearance) without modification or socket saver required.
I have a used Westinghouse from around 1967, and a brand new Psvane "247B" (it's really a 5U4G). I honestly cannot hear a difference between the 2 5U4Gs, but there is an improvement over the stock rectifier supplied with the MP-D2. The Psvane has about 50 hours on it. I'm under the impression it'll improve further after 100 hours.
The most dramatic change in sound came from swapping out the stock driver tubes with the BEL E88CC. The sound between stock and aftermarket tubes is like listening to 2 different pieces of equipment. It seems very sensitive to driver tube changes and I now understand why most owners have boxes of extra tubes to try out.
Hope this helps. Let me know if you have any other questions.