Thew important thing is your source...If you are coming from the keyboard via the mono output..there is no problem with a single input to the powered sub....but if you are coming from stereo outs of the keyboard..you should combine left/right to the single input of the sub..via a Y cord or change the output to mono from the keyboard...

The reason you need to have left and right combined to the mono input of the sub...The bass and bass drum may be panned hard left or right..this will prevent the signal to the sub that has only a left or right signal to the input..