From a November 2011 article found on mesaboogie.com:

"The core of the preamp switching and FX integration happens via Mesa/Boogie Amp Switchers controlled with Digital Music Corp.’s GCX Expander switching units. The Boogie amps being switched are two Triaxis Preamp’s run into a 2:90 Power amp. One Triaxis is set to the Mark IIC+ mode while the second Triaxis is used for clean sounds in the Green Clean mode. Like Kirk, James also has DBX Quad gates and T.C. Electronic G Major FX Processors along with a few extra pedals kept in the rack. The signal runs out of the 2:90 into two Mesa Traditional Straight 4x12s for a true stereo mix." (original source: http://mesaboogie.com/news/2011/11/metallica-live-rigs/)

Hammett was using this preamp as his main clean sound at some point.

In this video, John Petrucci's guitar tech, Maddi Schieferstein, gives a rundown of the guitar rig that was used during Dream Theater's 2014 "Along for the Ride" tour. The Mesa Boogie Triaxis can be seen at 0:23.

In this video, Tom talks about mixing the Triaxis with a Marshall JMP-1 for many years in tour with blink

In a video by Collarts titled "Ausmusic - Observing the Setup & Operation of a Stadium PA," a Metallica technician discusses the band's sound equipment, specifically mentioning Jason Newsted's use of the Mesa/Boogie Triaxis Guitar Preamp at the 26-minute mark.

From Musician magazine, October 1993 issue:

"Depeche Mode keeps things simple onstage: no amplifiers. Every sound, from keyboards to guitar to drums, runs through the P.A. system: a Brittania Pow Flashlight System. Dave Gahan and the backup singers use Samson Synth radio microphones with EU 757 capsules. Martin Gore plays a Roland A-50 and Alan Wilder plays an Akai MX1000, each controlling two Emulator Emax II samplers: Andrew Fletcher has another pair of Emaxes. Each pair is hooked up in parallel, so that if one were to malfunction, the other is ready. But 'I don't remember an instance when we had to go to a spare,' says Wob Roberts, the keyboard technician. Samples come from strange and sundry sources, including old analog equipment. The piano onstage is a Korg 01/W Pro X transplanted into a grand-piano body. For down-stage keyboards, Fletcher and Wilder use Philip Reese MIDI line drivers. A MicroLynx sends SMPTE time code to the video and film setups. Away from the keyboards, Gore plays either a Gretsch Country Gentleman or a copy of a Gretsch Anniversary guitar, strung with Gibson strings, from .010 to .046 gauge. Dick Knight copied Gore's original Gretsch for stage use, using Gretsch parts but adding more wood in the body to cut down feedback. The guitars run through a MESA/Boogie Tri-Axis preamp and a Zoom 9002 effects processor, with a Sennheiser UHF transmitter. Wilder's drums are mostly Yamahas: a 22" bass drum and 12", 13", 14" and 16" tom-toms. He uses Noble and Cooley piccolo and 7" snare drums and Zildjian K cymbals: a 22" ride, an 18" China, 16" and 18" crashes, a 6" splash and 13" hi-hats. And don't forget the tapes: two Sony 3324s, one of them a spare. Of the 24 tracks, Depeche Mode uses only 14, because many of the songs were dubbed from a 16-track Tascam that used 12 tracks for sound and four for sync. 'As soon as anyone sees the size of the machines, they think the whole show is on tape,' says Roberts. 'But it's just bass and drum parts and a couple of sequences. This band does not mime.'"

In his YouTube video titled "Mesa Boogie Triaxis - Quick Test," Ola Englund demonstrates the use of the Mesa/Boogie Triaxis Guitar Preamp.

ACOUSTIC GUITAR MAGAZINE – JULY 2006 ISSUE

Amplification: Fishman Ellipse blend (Martin); Mesa Boogie/Simul-Class 2:Ninety power amp, Triaxis preamp

For some different distorted tones, James has used a Mesa TriAxis all-tube pre-amp, which has 8 pre-amp modes, MIDI control, etc.

Exclusive gear from the private collection of Richard Z. Kruspe. Has been played live or in the studio with Rammstein and Emigrate. Every item comes with a certificate of authenticity, signed by Richard himself. Mesa Boogie TriAxis All Tube Preamp, used by Richard Z. Kruspe of Rammstein & Emigrate on tour. This is a US version that only works at 110V. For buyers in other regions, a transformer is required. A signed document of authenticity is included.

“love my Marshall stack and my Mesa/Boogie TriAxis—but for my live tone, I can’t beat those Sunn heads”

Max used a Mesa Boogie with Nailbomb. (picture taken from Gloria Cavalers's website)