I'd go for the line input on the Mackie - it will still go through the same preamp. So you will need a cable with a 3.5mm stereo connector to 2 TS jacks and only use one of the jacks if you just want mono.
If you really want to go through the XLR connector you will need something like a 30-40dB attenuatorin line with the signal - but as that's what the line input is feeding anyway you might as well use the attenuator built into the Mackie.