Lula, JWM and I have made the same exact "upgrades" to our MW's as you; Audio Magic, new umbilical, NOS Mullard and Tungsols(black glass) and each change made for serious sonic improvements and were all worthwhile. That said, the "stock" MW still sounds fantastic.
You may well be right about the additional break-in time needed for the PD. It really does sound pretty ordinary at the moment but JWM has it running 24/7 and we're waiting to hear what happens. One of us will no doubt report back.
Dave_b, if your MIT power cord made a significant sonic improvement, that's great but in my experience and the experience of four 'phile friends who own very expensive systems, we're all in agreement, the sonic changes made by power cords are subtle at best and often not for the better.
All but one of us have dedicated 20amp lines powering our equipment. I have four; one for each mono amp, one for digital source and one for analog source. We've all tried various cords, some quite expensive, and often, we try them in each others systems. As I said, the consensus is, the changes are subtle at best.
My suggestion to anyone considering a power cord change, upgrade your power first. What I find interesting in your case, you spent more on your MIT power cord than on most of your other components. Some might find that odd.