Blackstar, when you plug the XLR connector into the Axe I/O, does it automatically select the MIC LED on the front panel of the unit or is the MIC or LINE input selection done somewhere in the interface controller software. I didn't readily see a way this gets set either in the manual or from the image I found of the interface controller software, thus I'm thinking it senses an XLR and assumes a mic is plugged in(?) I'm assuming the MIC LED is illuminated for you and not the LINE which would likely result in much less gain(?)
The -37dBu signal to attain 0 dBFS seems like it wouldn't be adequate for the SM57 unless the amp was cranked and the mic was right at the amp. A typical condenser mic likely would have the needed output to drive the Axe input OK I believe.
The SM57 has a sensitivity of 1.6mv/PA or -56dBv/PA. The dBu equivalent is about -54dBu. I think the Axe would need about 17 dB more of gain to get to 0 dBFS in the interface. I'm thinking that trying another interface may prove this out and not a problem with the mic or cable. Guessing whatever interface you may try will be in the ballpark of 54 to 56 dB of gain.
FetHead would work if you find you do need a gain boost.
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