Hi Charlie, 

Not sure why the Mackie BlackJack Mic input works best with your setup when the XLR out is set for LINE level.

If the MXLNK pre-amp gain is set on the 'lower side' (ie. not peaking) it might explain this lower level at the MXLNK XLR out.

I'll take a look at the BlackJack user guide and chk their input specs.

thanks,

joe – eventide