Lets try to keep things separate, since your noise/high frequency phasing issues are being recorded to digital/USB outputs, that won't be related to an output stage balance. And the guitar input stage is all mono so not relevant.
Quote:
Originally Posted by Judkins401
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I checked the main outputs with a multi meter and found the left main XLR is always 3, 4, 5 mV stronger than the right
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A few mV difference is likely insignificant at usual output operating levels.
A slight imbalance of a few mV may not be a sign of problems. What I suggested elsewhere (that may have gotten lost in DUC problems) is you measure the outputs at higher levels, like 1 V rms, 500 mV rms and see how different they are. And (if they are significantly different) measure the +ve and -vs signal levels on each channel to ground as well, a significant difference there points to analog output stage problems.
The output levels you probably want to be working with should be higher, around hundreds of mV to ~volt, you should be adjusting down the gain in your powered monitors to accommodate that input signal level, and then play with gain staging for what sounds best.