It’s a common starter mistake to NOT set the volume in the DJ software and only use the gain on their controller, leading to (too) low output signals. I use Mixvibes Cross which has the volume for master and cue at the top of the screen, not sure off the top of my head how about Serato.
Lucky for you the SB has channel volume level meters and trim to help you figure the correct levels out.
* Make sure that at the loudest part of a track playing you will have all green leds lit and the orange one only on during peak signal. Move the trim knob til you have that situation set. If you can’t get enough signal on the meters, even with trim full open, then it is time to start looking for the master and cue volume in the software.
* Once you have the trim set correctly, keep the track playing.
* Set the the crossfader to the correct side (if active).
* Turn the master fader up. Don’t go beyond 75% open. I don’t think the SB shows master volume level, so no way to check there.
* Keep the volume setting on the speakers at 0dB.
You should now have the maximum available power coming out of your speakers without chance of distortion/clipping (potentially dangerous to your speakers and it sounds like crap).
Hope that helps some.