The AM2 is a replaceable/upgraded amplifier unit that you can use to change the sound quality of your FiiO X7. The AM1 (that is included with the FiiO X7) and AM2, have identical dimensions.
Where the AM1 has a brushed titanium appearance on the front and powdered titanium appearance on the rear, the AM2 is a slightly darker shade, and powdered finish on both front and back. Replacing the modules is extremely easy; simply using the Screwdriver that came with your X7 it is merely a matter of undoing 2 screws to remover the AM1 and replace it with the AM2.
The biggest difference with this amp is really the voltage output, and slightly higher output impedance with the AM2, which is still impressively low 0.5 ohm. As far as power goes, the AM2 is able to produce triple the output into 32 and 300 ohms. It also has a higher peak voltage output. The FiiO AM2 unit comes in a metal protective box which will protect which ever amp you choose to store in there.