I grab the backtrack (mp3), load it as an audio track in reaper. then i plug in my Mustang amp via usb. Computer recognizes it as a microphone. I select that as my input in for my track in reaper.

I turn monitoring off my my guitar track and un disarm my audio track. Then I hit record. It plays the audio track and records direct input from the amp with whatever presets I am using the FUSE software.

Then when I want to hear it, I just hit play and it plays both tracks. Works pretty well.

I am on PC, win7.

The kicker was to not try to listen to guitar playing thru reaper while recording. The delay is terrible, but it does not record with the delay, only outputs the delay. I listen my guitar sound from the amp itself when recording.