The digital audio is generated through Direct Digital Synthesis (DDS) using 512-value, 10-bit wavetables (-512 to 512).
The core DSP is in mono, the FX is in stereo. This saves a lot of processing and sounds just fine.

I've tried to keep the actual DSP code as simple as possible and the Due seems to not break a sweat at 10-note polyphony, so I can probably push it a lot more still. If you are experimenting with my code feel free to try this out yourself :)

The Casio CZ synths have a couple of oddities that I've implemented in Dradique to get that CZ vibe, though I did add my own flavoring to the recipe.

The DCO:
Dradique technically has 20 oscillators, however I made each DCO in a way so that it can generate 2 phase distorted waveforms by combining both the two carrier and two modulator wavetables into two wavetables twice the size and then iterating with a single counter.
The reason I did this is because of one of the oddities that the Casio CZ synths have: when layering two DCO's the signals are actually not mixed together, which is the usual way of doing it.
Rather, the each DCO plays it's cycle one after the other. I.e. first one cycle of DCO 1 plays, then one cycle of DCO 2 plays, then DCO 1 again etc... This creates an interesting sub-octave effect when certain waveforms are combined.
Dradique just does this all the time, allowing for easy realtime mixing of waveforms to create all sorts of vibes, while limiting the actual calculations needed by combining two DCO's into one.
The Dual mode in Dradique actually layers two DCO's normally, mixing their signals together and tuning the second DCO down one octave.

The resonance in the CZ synths has nothing to do with phase distortion at all. It's nothing more but a sine wave at a frequency relative to the DCW "cutoff" frequency, hard-synced to the DCO. This sine wave is amplitude modulated by the inverse of the oscillator's phase. This is called using a 'window function'. This also means you can use a waveform as the window function, thus creating various resonant waveforms. In Dradique I just used the phase as the window function since that sounded really nice.

This picture from WikiPedia illustrates the process:

Here's the struct and DSP code for a single DCO:

//'oscs' is an array of structs
//here's the struct:
struct DCO
{

//property values, used in main loop
byte note;
bool gate;
bool env_stage;
bool mod_stage;
bool busy;

int16_t envelope;
int16_t modulation;
int16_t resofreq;

//processing values used in DSP code
volatile uint16_t freq; //phase increments
volatile uint16_t counter; //phase counter

volatile uint16_t resocounter; //CZ style resonance

volatile int16_t waveform[1024]; //carrier waveforms * 2
volatile int16_t modulator[1024]; //modulator waveforms * 2 (distorts phase)

volatile int16_t mod_depth; //modulation depth 0 - 63
volatile int16_t env_val; //envelope value 0 - 63

};

//and here's the DSP:
//increment DCO phase
oscs[0].counter += oscs[0].freq;

//create an index offset in the phase and use it in the wavetable
uint16_t index = oscs[0].counter + (oscs[0].modulator[oscs[0].counter>>6] * oscs[0].mod_depth);
int32_t result = oscs[0].waveform[index>>6] * oscs[0].env_val;

//only update the volume when the amplitude is small, this reduces zipper noise a lot
if(result < 1024 && result > -1024)

oscs[0].env_val = oscs[0].envelope;

outvalL = result;

//resonance output, only for the second waveform, CZ style :)
//so keep the resonance phase at zero the first half. This also creates the hard sync
//that is needed for CZ style resonance
if(oscs[0].counter < 32768)
{

oscs[0].resocounter = 0;

}
else
{

oscs[0].resocounter += oscs[0].resofreq;
outvalL += (tab_sine[oscs[0].resocounter>>7] * (65535-oscs[0].counter)>>9) * mod_resonance.value;

}

The DCW ("filter")
The DCW of an oscillator is the magnitude of distortion that is applied to the reading angle of that oscillator's selected waveform. The DCW can be modified over time using an ADSR envelope, thus changing the timbre of the sound over time. This acts much like a low pass filter cutoff hence the analog vibe of the Casio CZ synths.

In Dradique I implemented the phase distortion by actually adding a waveform to the phase incrementer itself, this creates the exact same effect of phase distortion as the Casio CZ do. Again, a picture from Electric Druid illustrates this really well: