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What is causing the hum when connecting my SI expresssion 2 main L&R outputs to Club PA system

This is a strange one. Just did a gig at a club that had a great in house system. 3 Way, Crown amplification EAW tops and bottoms. I could not get rid of this low frequency hum which sounded like a ground loop but was not that bad however it should have been silent as my setups are always crystal clean and quiet. I'll make the setup simple here. Just the si expression powered on with no channel inputs and not connected to the club system there was no hum in mains speakers but soon as I connected one or both XLR left and right Aux 15 & 16 there was noticeable hum in the main PA. I then checked a few of the other AUX sends with my wedges and they were crystal clear even while the hum remained in the house mains. Well I thought lets try my main out Aux 15 & 16 to my wedges to see if there was an AUX issue and no they were silent. This confirmed the issue was definitely between the console and the inputs to the club system. So I tried all the troubleshooting tricks like, XLR ground lifts on both Left and right, temporarily lifting power cord ground, moving to different power sources, moving any power adaptors away from the console. None of that worked but here is something that did. I used a soundcraft analog mixer that was there to feed the mains and there was no hum at that point so I then plugged my si expression into 2 spare channels of the analog mixer and damn it got rid of the hum. I am lost here, what the hell ? Would there be an output impedance issue or miss match causing this ? I know you can't always depend on club systems because we don't always see how they are wired or what quality but the thing is it was crystal clear as long as that analog mixer was used in series with my si expression. Just doesn't make any sense to me. Any suggestions ?

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