What tms said, including the weight problem (and maybe the lack of a pan pot for the center input).
The main issue when using this preamp with professional equipment is, if you have microphones with balanced outputs, how do you unbalance their signals at the inputs to the MX-100? The right answer may depend on what type of microphones you're using. The only universal solution is a set of good input transformers, and those aren't cheap--but no one other method will work with balanced signals from all the different types of microphones that are out there.
Still, despite its somewhat unusual personality (it fits precisely with a "recording philosophy" that Nakamichi used to advocate), this is an amazingly quiet and overload-proof little preamp. I have one that I rigged up 25 years ago with stereo phone jacks (for connecting balanced microphones) and a set of Beyer miniature input transformers, plus 12 Volt phantom powering derived from the main DC input socket.
The main problem was, in a live situation you can't always judge the level setting that's right for the center channel input, so you get stuck with whatever balance you guessed at the time. That's why RCA and Columbia used to record a lot of stuff on special three-track (half-inch) Ampex recorders--they could choose the best settings later on, in the comfort of the studio where they have known good monitor loudpeakers. Headphones can't really tell you what you need to know in a situation like this.
--best regards