Hi Artem,
You are correct in that the ZDSD is like having a built in ZBIT.  However, if you add the ZBIT externally, you have the additional benefit of signal attenuation allowing you to dial up, and dial back the voltage which is a wonderful feature.  If I were you, I would add a ZBIT to your DA-3000.  

With regard to your distortion question, it is hard to comment on without knowing what is causing the distortion you hear.  Just to be clear, are you simply hoping to improve the sound?  If so, what it is that your sound in lacking that you would like the ZBIT to solve?  

Your amp, run unbridged, has an input sensitivity of 1 volt.  Most source equipment running unbalanced has an output voltage of 2 volts.  This means that your amp will reach its maximum output at a lower volume setting on your passive pre-amp when sending the amp a 2 volt signal (or much higher voltage with the ZBIT).  

Regardless of the distortion issue, there is no doubt the ZBIT will improve the sound potential of the DA-3000.  However, I cannot say for certain how your particular amplifier will react to that higher signal voltage.  

Thanks, Jeff!
By reaching distortion area i mean that the amplifier is already overloaded by high voltage (because amplifier inpuit sensistivity is quite small). So may be i don't need additional voltage - it could be a benefit for somebody else with a very low voltage source. Trying Zbit is expensive option because i should buy also good XLR cables.
I don't know what i am lacking - i just hope to improve the sound.

Jeff is right. I know it's an expensive option, but there's few other options like it, and it makes every component it's attached to "better sounding." I have two, and will one day have one more. . . .