you want to be using a mono output, so just use the left output, ideally line out not headphone output, the re-amp box will then sort the impedance mismatch and turn it into a high impedance signal output, you will need either a stereo re-amp box or two mono re-amp boxes to do it in stereo. The other alternative being do two runs one for the left, then one for right and blend them together.
is your interface a 'line out'? what type of output is it?