Slight saturated peaks
There's some Saturation with the instruments as they reach loud or crowded peaks between each-other. Aside from that, this song is pretty awesome.
I only know two ways you could fix this, it's hard to tell them apart sometimes so I'll list both.
If you can, place a limiter or compressor on the main mixer channel.
So if it's just volume issues,
Here's the usual compressor settings I use;
Type = Soft (if your compressor has this setting)
It should make the softer sounds a little louder while not allowing high end distortion
Now if it's instrument cramming causing this, you'll want to use an Equalizer(EQ) on each instrument. I recommend a Parametric EQ if you can, it's easier to see what you're doing.
Then you have to go through and decrease frequencies that class with each-other on each instrument. By Watching the spectrum visualization here on newgrounds, I'm guessing it's somewhere around mid-bass area from frequencies 230~560 from the bass, pads/reverb and chromatics.
I hope I was of some help to you.