It looks like you're using PWM and a lowpass filter to output the audio, I'm using the same kind of thing for my synth.
At ~32KHz sample rate, that's about 500 step resolution for the PWM. (I would guess it's actually 512)
How are you getting the "headroom" for 8 notes 8 bit waveforms? Do you just have the normal volume of each oscillator at 25%?