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The unit has balanced inputs AND outputs. Would it be best to use an XLR to 1/4" TRS patch cable?
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Ideally with REW, you want to create a soundcard calibration file that is the exact inverse of the combination of input and output circuits you'll be using for measurement. That way, the audio card imitates a perfectly flat response and doesn't influence the room measurement.
With PCI type soundcards we use the line-out looped to the line-in because those are the connections we use to measure with.
In the case of the M-Audio cards where you're using the phantom powered microphone pre-amp as part of the measurement, it is not really possible to include the preamp in the loopback test to produce the soundcard cal file. But, no big deal really in my opinion, because we will have to assume the preamp has a perfect response. That is certainly what we do when we use an ECM8000 with the EURORACK UB802 (which actually does have almost a perfect response for the frequencies of interest).
So, I believe what most people do to create the file is to loop the line-out to the instrument line-in. Since the instrument line-in appears to accept unbalanced 1/4" TS connector plug, then I would loop that to the line-out that you'll be using as part of your measurement (I assume the RCA unbalanced out #1 or #2). So you would need an RCA to 1/4" TS cable for the loopback. Then for the measurement, use the RCA #1 or #2 line-out and the XLR balanced microphone jack for the ECM8000.
brucek