When I read the title, I assumed it was like a 12 string bass with a low B added. Since I've worked with Joey DeMaio and Bill Dickens, I'm very familiar with Joey's 12 string bass (3 strings per note) and Bill's 9 string bass (added F# below the B of a 5 string plus another three higher strings), but this is just silly.
Most guys that play these types of things have very bright sounds, so the amount of low end isn't what you would think it was. A normal clean bass amp would work just fine.
I saw Bill Dickens play live once (with Neil Zaza). Anyone that can temporarily steal the show from Neil Zaza like he did is top notch. I've never seen a bass player play like that...it was glorious.