Here's my two cents. . . .

If your DAC output is 2 volts single-ended or above, I don't think you really NEED a ZBIT. I bought a ZBIT because at the time my DAC output was about 1.3 volts single-ended, and I needed the 2.6 volts or so balanced XLR output for my Decware amps, using the DAC volume. When I moved up the chain to the manufacturer's new model DAC and its single-ended voltage output was 2 volt I really didn't NEED the ZBIT any longer but my best cabling was XLR from the DAC and it made sense to just keep the ZBIT in place. It still is and I really like it, but if I had one more set of the exact same single-ended interconnects (which cost more than the ZBIT) I could do without it I suppose. So if the voltage from the DAC is high enough I would say it's not a necessary addition to the system.

As for a preamp--the SEWE300B / Sarah (for some reason I don't like to call it Sarah) amplifier has in my opinion the perfect preamp stage built into it and I used a fully-modded CSP3 preamp for a while but found it was just not necessary--it created a super dense sound, and I love the density of my system (with a ZROCK3) but prefer the sound of the system without a preamp. So if you have the requisite voltage and your DAC remote volume control is great, you can forego a preamp with the SEWE300B.

Just how the implementation of the SEWE300B has evolved in my system over the last two and almost a half years.