I had a V1.38 many years ago, that had an EPROM update to V2.01...and now I own an original V2.01 since some months.
Firmware version on EPROM has nothing to do with how the MP-1 sounds...and with its tone.
It's more for MIDI, factory presets and so on.
Don't know what Michael Wagener smoked or drank at that time, but for sure it was something strong.
By the way, aside from EPROM, my current MP-1 has the same character and tone of my previous, older one.
I just suspect the V1.38 (I bought it used) had some minor issues with gain...probably some components on the tubeboard...because with OD1 and OD2 at 7.0 I now get more driven tones than the ones I was used to get with the previous one with OD1 and OD2 at 10.0 (I used PM and GT 12AX7/7025 tubes at that time, now TAD and Ruby selected JJ).
My 2 cents.