This thing is ridiculous. I read somewhere that more albums have been recorded to an Ampex 351 than any other tape machine. Elvis, Sun Studios, and the like. Google it; it's an American icon.
The really fun thing to do is use the preamp as a tracking preamp, bypassing tape. There are some mods to do. I've tried the Altec 1567a tube mixer, 2 different Grommes Precision tube mixers, some homemade ones, etc... None of them come close to what the Ampex can do. It ain't flat, but who wants that? A ribbon mic into one of these is a huge surprise. I can get crispy top end on an acoustic guitar this way.
Crazy, my experiments with ribbons have often left me wanting more top end if it was pointed at a guitar or voice. The Beyer 160 is an exception, I use it everyday on drums, but a Royer 121 is a dark, chunky thing typically. Through the Ampex, the response is so colored, I actually attenuated top end when I tracked slide acoustic guitar.
As of this writing, my go-to signal chain for guitars, bass, mono room mics, voice, piano, etc. is the Ampex into a Fearn EQ, then a Inward Connections tube limiter. This is an expensive route for sure, and it sounds like it.