just means single vs dual MM style pickups.
typical nomenclature is M (single M, like the original model), J/M or J/MM (like a Lakland) or MM/J like a bongo, or MM or double-buck (warwick corvette with dual M's in it), etc.
it gets confusing because MM can mean either MusicMan (as in single MM style pickup), or Musicman-Musicman (dual) depending on context.
Gotcha. I never really understood the appeal of an Ernieball bass outside of the classic stingray setup (meaning one pickup, not the ‘classic’ series, althougth I LOVED that iteration) and the ones I’ve played have left me in ‘meh’ land.
Between the feel/ergonomics of my P-bodied NYC’s and the simplistic-yet-imminently-versatile Sadowsky pre, I feel like the bass I got is basically the dream Musicman that Ernie Ball and CO will probably never make.
I had one of the boost-only ‘Classic Series’ basses, but again, the spacing thing really bugged me. That and it had a weaker sounding D/G overall, which IME is fairly common on musicmans, but is NOT apparent on this bass I just got.