My guess is that you won't get the required accuracy by modding the device. Try Ian's "Neve Trick":take a 1k variable trimmer and put the hot leg of the 600 ohm transformer secondary to pin 1 of the trimmer, take the "wiper" or pin 2 as output and connect the TX 600 ohm secondary cold leg to pin 3. Do that on both L and R and set the trimmer to match gain.
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