The X73 has two individual input transformers, one for mic input and the other for the line input.
The X73i, 273, and 473 have one input transformer and a button on the front panel which switches the transformer to either work with the line input or the mic input.
Sonically they are the same preamp made using the same transformers.
The AMS 1073 DPA clone also only has one input transformer that lives on the mic input, the line input on the other hand is balanced by an opamp and not a transformer like one would expect.
