I very much agree. I would like to see a FAS "model 1969" Marshall (Major, or year 1967 Super Bass 100, ~Williamson clone.) It is both a great amp and a terribly designed and very basically flawed amp. The NFB loop was huge. It included a preamp tube, tonestack, Cathodyne PI, 12au7 driver tubes, power tubes, UL OT. It had terrible ringing due to phase lag. "Transient Distortion" per Matti Otala. A conjunctive filter (2n + 5-20K resistor) can damp that ringing just part-way down and, in so doing, produce some very interesting and unique overtones. Also, a conjunctive filter is much more effective with an UL OT. And, a conjunctive filter helps to remove some of the harsh high end of an UL OT -- along with a lot of hash and fizz. That sterile, super-accurate, UL high-end is generally unwanted in a guitar amp. After that, add a jumped 1st-Stage and the NFB component values from an EVH "brown" Plexi, and it starts to sound pretty good. And for good measure, add an adjustable cathode resistor to the 12au7 driver stage -- to create a MV/saturation control. And, make the conjunctive filter's resistor adjustable/tunable, too. The tonestack will still be somewhat limited in use and range -- due to the NFB loop trying to "correct" the tone settings. But, that is where some of the great low end resonances are created. And those resonances are where a lot of the fist-punch-in-the-chest bass comes from. Anyway, it is a flawed amp. And, it is pretty unmaintainable -- between the high voltage 560-660v and the NFB phase lag. And, that is particularly true if a squarish distortion signal is sent into the front of the amp. At full-bore, a trifecta like that would produce an almost 3K transient voltage spike. So, big expensive UL OT's blew.
Today, in real life, the only tubes that can handle that kind of high voltage, 560+vdc on the screens, are mostly just a few of the NOS tubes such as Mullard EL34's, at $250 each (last time I looked.) The "model 1969" Marshall Plexi, built in 1967, can no longer exist as a working man's amp -- due to it's very extreme and also very poor design. In fact, it barely existed for a year as an amp, in 1967, when Marshall "created" it -- due to it's unreliability. But today, it can exist very reliably and become an even better amp in the Fractal Universe -- kind of an Electronic Jurassic Park. Or, maybe, I am thinking of an electronic Geriatric Park. I really don't know. I might be too old to know for sure.
. However, the 1969 model (1966-67) never really belonged in the analog world. Which is why, it was constantly trying to pass into the next one. 
. Some of the best death metal tones were out of weird multi fx units you wouldn’t expect, from the likes of Korg, Zoom and Digitech.
