“I'm confused. I have a condenser mic (Rode NT1A) with a separate preamp unit (VTB1). Can I plug the Rode into the preamp than the line out from there to the line in of the Motu?”


Yes, you can. Be aware that you are introducing another link in your signal chain using the VTB-1 though. It will probably introduce some noise into your signal. It's a great little pre (I have one) for the warming effect of the tube, but it's not the quietest box around. Try it both ways.


“And can my dynamic mic (SM57) go into a line in? Or should all mics go into a low impedance input.”


The 57 is a low impedance mic. Use a low impedance mic input.

“Impedance Rated impedance is 150Ω (310Ω actual) for connection to
microphone inputs rated low impedance.”


http://cdn.shure.com/specification_sheet/upload/81/us_pro_sm57_specsheet.pdf



Regards,

Bob