Reentrant C an octave down. Hmm, yeah I guess it's possible, but it will be a challenge finding strings. Fro my reading, it seems like the Aquila 23U is standard high g tuning which is G4 C4 E4 A4. If you really meant an octave down for all strings, I agree a baritone scale will be a must and you're probably going to have to get a special nut to accommodate the thicker strings.