Two of the 9 volts are wired in series, providing 18volts. Two are wired in parallel.
I don't think you can wire 9volt wally batteries in series to make 18v. I believe the 9volt bats are actually 7.2 internally and have regulators which raise to 9v. So this is pretty tricky.
Right now, I can go tape at a local hole and just run constantly from 10PM to 2AM. But with the nbox and the main gig coming on around midnight, there was always the concern that I might come up short due to recording the openers. Especially if I was starting even earlier to catch soundcheck. So it isn't just festival tapers.
The battery wires in my nbox were quite short. There was always the concern that one would get damaged at the worst possible time (somewhat drunk (or not) while changing batteries before the main act, etc). It was nice that the connectors securely grabbed the battery terminals but they were Really secure. I found it much safer to pop them off by inserting something rather than trying to pull them off with my fingers.
Without batteries, the nbox could be less than half the current size. I think a half size nbox with sep bbox would be easier to conceal then a combined unit.
I'm not trying to rag the nbox, just trying to think of ways it could be improved. One question is how current units could be improved and the other is how a new design could be improved.