Hey BJ,
There's nothing regrettable about having the one that started it all.
The 2.01 firmware is the last update to the MP-1 while it was still actively manufactured. All that did was add a few more factory presets into the permanent memory, but it was basically still the same preamp that it was when it first appeared in 1987. You can still find ADA MIDI controllers, they do pop up every now and again, but bear in mind, The ADA's use a 7-pin DIN cable, and you need the ADA wall wart to plug in to the back of the preamp.
Realistically, you can plug in any MIDI controller with it's normal MIDI cable and it will switch program patches for you. You just won't have the phantom power option.
I have a couple of those ECC803's in one of my preamps, they aren't bad at all.
Either way you go, you've got one of the best preamps made, and you're running it through a tube power amp section, so you've also got the tones!
Enjoy
Harley