Casio CZ/ VZ/ FZ - Pro Series

>So, could someone out there tell me, if you crank up the gain on your
>box' line level outs, do you get the noise also? If so, then I would
>suspect that this is just a common trait of these synths.

Mine is almost completely silent when not playing any notes, however this 
seems to be from a muting circuit that cuts off the output when the signal 
to the DAC drops to zero, like a noise gate (I haven't checked the circuit, 
only noticed the effect of it).  There is some noticeable quantization noise 
and hiss otherwise but I've never found this to be a problem at all in the 
context of actual use.

I've noticed that on mine, the headphone output on the front panel doesn't 
sound as nice as the line outs (though it may be a result of running the 
line outs through a mixer), the quantization noise is considerably more 
noticeable.

Make sure you've got any internal volume settings at the maximum (operator 
levels, patch levels in patch split/layer or multi-channel modes, etc.), 
anything less reduces the signal to noise ratio.  And remember that ideally 
you should keep the mixer levels close to unity gain to avoid introducing 
additional noise.