BG's experiences with his Zons really concern me, as I have recently been very interested in acquiring a Sonus 4 lined fretless - probably used, as I can't afford a new one right now - the one I played (wasn't for sale, btw) had no deadspots in the neck and had "The Sound"...
However, there seems to be be far too much "caveat emptor" going on:
the maker knows there are occasional problems with the necks but refuses to put things right if the instrument is off warranty and/or purchased from a nonZon-authorized dealer, w/o a fairly substantial cash outlay by the owner of the bass.
That's not customer service no matter how you slice it, esp. in BG's case where he has spent considerable coin with the maker.
Makes me think twice about getting a used one, if it's a roll of the dice re: neck issues and maker-service.
For-sale used Sonus 4 lined fretlesses seem to be rare as hen's teeth as it is, w/o the worry of possible neck issues...
JMHO