I find it interesting that we live in a time where amp/ pedal builders are trying to add their own spin to classic circuits, but no-one has tried to make a clone of the MP-1. I'm aware that the electronics are different from that of a Plexi, but with the schematic, knowledge, and time, it's probably doable.
I beg to differ. There have been many followers since the introduction of the MP-1 in 1987, and all of them have been well known and established companies that have been around for a long time. In 1987, we only saw the ADA MP-1 as far as rack mounted preamps, but in the following year, Fender introduced a solid state rack mount two channel switching preamp based on it's R.A.D. combo amps at the time, and Marshall also introduced a solid state rack mount preamp, based on one of it's combo amps at the time. Neither one impressed anyone. Also, Rocktron and Scholtz R&D brought out their first rackmount preamp offerings. Soldano also introduced the X-77 switchable two channel preamp based on their SLO-100 preamp section. By 1989, every amp builder had a rack mounted preamp on the market and each one of these had their own spin on what a preamp should be.
Ultimately, the trend for rack gear fizzled in the 90's as it required some thought to select and build a good quality rack amplification system, which unfortunately, most guitar players hate to do. Marshall ultimately developed the JMP-1, which was their best selling rack mount preamp. Fender on the other hand did develop a prototype tube preamp, and integrated tube power amp, ( RP-1 and RA-1) in a rack mounted format, but they didn't make it to production. Instead, Fender opted to stay with something they were best known for in amplification and that led to the introduction of the Cyber-Twin Amp. Basically, it was a programmable preamp using two 12AX7 tubes, with the ability to store amp tones and effects into preset banks. The main difference being you just dialed in the knobs to get the sounds you wanted, and saved them. The user interface was just more friendly with the amp knobs instead of membrane switches like we use on the MP-1, MP-2, etc.
Once again, even though this was easy to use, the purists like to argue that it's not a real amp, so even that was discontinued a few years ago. So you see, they did build preamps, and amplifiers based on the MP-1 designs with their own spins on them. It's just not a popular choice with the simple-minded guitar player who wants a guitar, a cable and an amp, and just turn it up and go.
Harley