In the YouTube video "The Guitars of Metallica: Kill Em All era" by CIRCLE OF TONE, it is discussed that James Hetfield used a Marshall JMP50 Plexi Amp Head during his early days with Metallica.

Dimebag Darrell used the Marshall JMP50 Plexi Amp Head, as shown in a user-uploaded photo.

This GuitarWorld article states that on Superunknown, Chris Cornell’s tone came from a blend of a Marshall JMP50 head (not the combo which was previously submitted) and a Mesa Boogie Dual Rectifier. It is quoted from this article; “Most of Cornell’s guitar tracks were recorded using a Marshall JMP 50 half stack and Mesa-Boogie Dual Rectifier Solo half stack blended together. On Black Hole Sun, the Marshall provided brighter tone with defined string attack, while the Mesa delivered darker tone with bigger body and bottom-end girth, and both amps were set to moderate levels of overdrive.”

According to a Guitar Geek rig diagram, Mike McCready used the Marshall JMP50 Plexi Amp Head.

"Taking a look at the other amps in his repertoire – such as a ’59 Fender Bassman Reissue, a Fender Vibro-King 60 watt combo, a ’69 Marshall JMP50 Plexi head and a Marshall 1987X Vintage Series 50 watt tube head, among others..." - "PAL" Adam Stafford.

Kaukonen mentioned 1967 50W Marshall Plexi amplifier on his website, which is likely the JMP50. The amp was used for Hot Tuna's Last Waltz tour in 2023, according to 2023 Guitar World interview.

Jorma about the amp, (source:)

Moving right along. When it came to live with me it was covered in decaying white tolex and purported to be a metal face. It required some work by Dr. Amp up in Canton to make it functionk. He did the best he could do, but it was really boogered up. I was trying to make it reliable enough to use it full time on the stage with limited success. George Allessandro the high end amp builder in Philly came to see us play… he’s a buddy of Jacks. Anyway, he was appalled at the condition of the amp. Turns out he’s one of the leading classic Marshall amp guys in the world. He took it back to his shop and under all the junk found it to be a ’67 plexi. He replaced all the tubes, caps and pots with new/old original Marshal parts and recovered the the head with new/old original stuff. It now looks and plays like it did when it left the show room floor in 1967. I’ve got a lot tied up in this old beauty so it is not going gentle into that good night.

Photo taken of Torsten’s 1969 Marshall JMP 50 in God Is An Astronaut’s Recording Studio

Gustavo Nápoli uses the Marshall JMP50 Plexi Amp Head, as shown in a user-uploaded photo on BlogSpot.

According to a list of equipment which is used in his studio, Crowdspacer, Joakim is also a user of Marshall JMP50 plexi amp head.

Assuming its the selected amp but not quoted as to the exact version:

"Aside from firing up a cornucopia of living-the-dream amp heads, including a Marshall Plexi, Diezel VH4 and Orange Thunderverb, Joe’s other great discovery on All That Divides was a quick-fire alternate tuning. While he usually opts for D standard, with the odd drop-C sledgehammer thrown in, he explains that one small tweak was behind a number of the album’s highlights and his now-trademark chord drones."

Brent states a plexi JMP 50 is in his amp collection.