[Audioarchivist]

standard turntable outputs are similar in level to microphone - except for grounding the TT if you can adapt the rca's to go mic in you might be all set, but a TT preamp is an easier way and probably better.
I have gone TT out rca > 1/4 inch adapters into mic in, but that mbox has xlr mic in, line in 1/4 and DI 1/4, so I dunno 4 sure. Ground wire (if your TT has an external ground wire) could attach to something metal on the mbox or to the outer ring of one of the rca's - just listen for no more buzzing hum when it's on right.
With most mic pre's there's some sort of trim pot or pad that will let you adjust levels to boost or cut enough to bypass a TT pre altogether...