Connect the amplifier to your digital audio interface using the line cable, the line-out port on the amplifier and the line-in port on the interface. Ensure that the amplifier is powered off before making these connections.
Plug the audio interface into the computer using a FireWire or USB cable, depending on which connection method the interface utilizes. If you are using a newer Macintosh laptop, you may need to purchase a FireWire 800 adapter plug for your FireWire cable.
Power on the computer and open your preferred digital audio workstation, or DAW. Programs such as Logic, ProTools and Record are equipped to read and play MIDI files using their own collection of synthesized sounds. Open the MIDI file using the "import file" or similarly worded command within the DAW.
Select the audio interface as the preferred audio output device within the "audio i/o" settings menu of your DAW. The DAW will now route all playback directly to the audio interface and amplifier.
Lower the volume on the amplifier and power on the device. Slowly raise the volume level, listening for any abrasive or unwanted sounds. If the amplifier appears to be functioning properly, raise the volume level to a comfortable level.
Press "play" within the DAW's transport panel. The amplifier should now output your selected MIDI file.