Not sure if the fx send of your Trace will do parallel out but the tuner out will.
If you get a dropout of your low end it is usually caused by preamps outputting on opposite phase.
Or one of the cabs wired in reverse. Before you get too far along you should probably battery test them for polarity. + on the tip of jack lead makes speaker cone jump outwards.
Other possibility with Trace is the fx loop on some can be set to run without any lows in it ''high pass'', don't want that.