[brucek]

It would be best to include as much circuitry in the soundcard correction as possible.

So, if you intend to use the mic preamp for input and the 1/4" balanced connection for the output, that would be the best route to use for the Left Channel loopback. But, that would be problematic I think, because the mic amp will offer fairly high gain and would require some precise level adjusting to get the looped back channel to behave with REW.

Myself, I would probably use the soundcard cal file method. Install the loopback on the right channel and play with the levels until you get a good cal file, then simply use that for the calibration and forget about the left channel cal.

brucek