Not being a bass player, I'm not familiar with what a SSB bass head is. Single Side Band? Same Sex Bass? Dunno.
Wait, I just went and looked - must be Short Scale Bass.
I don't know why they wouldn't offer different nut widths on those, but I do know that the Warhead is a derivative design. In the guitar section, you can have any Strat neck re-cut to a Warhead for $45. I wonder if you called them they could do the same thing with the traditional Fender bass heads? it would widen your selection dramatically. Worth a 2 minute call.