First of all I have to admit that I didn't read the whole thread, lazy me.
That being said, here's what opamps I put in my MP-1. Basically I swapped U4, U11, U15 and U18 for OPA2604 (with sockets). According to my notes U4 was originally a NE5532 and the others were RC4559N. I kept U9 as is (almost, it was an LF412CN and I put an LF412CP, because I mistakenly swapped it for a TL072 in the first place and damaged the original opamp when I removed it).
I chose the OPA2604 over the OPA2134 because some said it was more tube-like whereas the other was more hifi, whatever that means. I'm sure the 2134 wouldn't be a bad choice either.
The bottom line is that I am more than pleased with the results, it's as if a blanket had been lifted (that's so cliché!) and I suddenly had some nice high end coming back at my disposal. And of course the noise went drastically down (but I also changed the caps as per the noise mod instructions).
I also felt that my gain went up too. Previously I had doubts and was considering a gain mod, but no longer now, I get the gain I need, and more (think Bettencourt gain).