After much discussion on the Ampex listserv the consensus seems to be that the mic inputs on Ampex record electronics were never intended for serious use. On the 350/351 recorders, 99% of the time users would run mics through an external preamp, and usually they'd bypass the input circuit on the electronics. (On the 351, at least, this padded the line-level signal down, then ran it through the not-particularly-good mic preamp.) The mic preamps were basically for emergencies, cue mics, etc..
So what are they? In the case of a 351, a 1:10 (I think) transformer connected to a 12AX7. Too much gain for most condenser mics, too much noise for dynamics and ribbons.
One of these days I'll dig the 351 electronics out of the basement and give them a workout on the bench and in the world, but not with a whole lot of hope attached.
They sure do look pretty, though.
Peace,
Paul