Soft, not clipping track distorts

I know this is a mixing issue, but I was wondering if anyone might have some insight into what I can do to help with this.
 
I've got this one sound which is sort of a keyboard/bass combo in Omnisphere. I've got this 4 bar rhythmic thing I'm playing with it that is really not very loud at all, doesn't even come close to clipping, yet on some notes, it makes my speakers distort. I can't hear this distortion on headphones at all. Just out loud on speakers. I've been trying to mess around with some EQ plugins to find where the distortion is but so far no luck. The closest thing I can do is turn off this compressor effect that's part of the instrument preset in Omnisphere, and then it clears it right up but makes it so low that it's useless. If I then use some other means of bringing the volume back up, the distortion comes back.
 
Any ideas?

Hmm, I'm gonna take another look based on your response, but that's exactly what makes it all so weird... Specifically in response to what you're saying...
 
1) I'm not maximizing any levels in Omnisphere. I'm playing with a preset included in the package and not changing any levels within it (until I'm already trying to fix this distortion, but that's after the fact).
 
2) The distortion is happening at the beginning of the project, when that one Omnisphere track is the only sound playing, and at most it's getting up to about -12dB (right when the chords hit), the rest of the time its between -24 to -18.
 
3) towards the very end of the project, a bunch more instruments from various synth sources are added and everything crescendos and gets up to like -6dB (this is a scene in a horror film). But at that point the distortion isn't even really noticeable as it's mixed into the wall of sound I guess.
 
So really the problem is when things are really quiet low and only one track is even playing. I suppose it's possible it's the way the creator of the preset within Omnisphere made the sound (since I'm not editing any of its parameters), but you'd think they'd test for stuff like that. And like I said removing the particular preset's compressor seems to remove the distortion, from the sound, but then anything i try to raise its volume back up brings it back.

Hmm that's an interesting thought... I'm taking a look at the control panel and all there really is is USB Performance Mode: Recording / Balanced / Mixing and I'm on Balanced. I'm going to try Mixing and see if it makes any difference... I've never played with those settings before.