I am intrigued, but I was not completely convinced by the sound files. I know MikeB recorded them somewhat roughly but the upgraded transformer seemed to have a bit more treble and this seems to affect the top end of the distortion quite drastically.
I like the idea of 12 v getting to the heaters and the increased anode voltage - though that can reduce distortion as well as increase headroom. Listening to MikeB's recordings made me wonder if it also made the MP! a bit more brittle.
I quite like the sound of my MP1, though if an upgraded transformer would give more 'body' I'd be interested.
I suppose the other issue with the psu is brown out - I have done a few outdoor gigs and if the generator supply drops below say around 180 volts, the MP1 shuts down... Now if that area could be made more reliable I would definitely be interested!
Perhaps someone here has sussed that problem and engineered a solution...