The CMC6s are mic bodies only - they still require a phantom power source and gain prior to hitting your ADC. The NBox is more multi-function. It replaces the mic bodies, provides power to the caps, and adds a fixed amount of gain. The NBox's fixed gain means you need to have an ADC with variable control over the levels - the NBox may provide too much, in which case you'd attenuate; or it may provide too little gain, in which case you're SOL unless your ADC also adds gain.
IMO the only reason to go with the NBox over the CMC6 bodies is for lots and lots of stealthing activity. Otherwise, the CMC6 + a good variable gain preamp sound better.