If my memory serves me true, there is a 'Sub' type preset available. Also, I believe the LP filter can be adjusted.
However, if you have a [pro, balanced: @ +4dB] crossover/equalizer hardware processor available, place it between the mixer's Sub output and XTI. Since the mix output will used for the stereo image, think of using an auxiliary send as the Sub output from mixer to crossover processing (either preset or hardware).
Use Ch1 as main input and set output mode to Bridge; connect sub network to amp (sub speaker load is now between the two positive (red) output terminals: more (twice!) power available as a mono block.