On the soundcard?
Well try and find the LOUDEST, busiest SMF you can, with the drums set to 127 volume, then add on several played keyboard parts, all set pretty loud, and have it play along with an mp3 or .wav file that has been normalized up to 0db.
See if you can make it clip... Most keyboard's are gained very conservatively for a reason. You never KNOW what is going to be played through them, and digital gear is completely unforgiving of even the slightest clipping, and can make all KINDS of horrible noises and intermittent playback...
If the headphone volume slider hasn't accidentally been set too low, and the arranger's main outputs are driving your mixer pretty well, personally, I would STILL not jack around with the default soundcard gain. It might not bite you in the butt most of the time, but there's a strong possibility that it COULD, depending on the circumstances. And a clipped soundcard is something NO audience should ever have to hear!