GeoffGould
Commercial User
Captain Pedantic says: They're both "Quantums" it's just that the neck-thru model is the "Quantum TBX".
Pretty sure "Quantum" originally referred to the body shape/style with that gracefully elongated upper horn. Lord knows there have been close to a dozen Quantum variants over the years, some neck-thru, most bolt-on (and some bolt-ons with the Integrated Neck Joint, others with a traditional Fender-style heel & neck plate). But all Quantums.
This has been covered before, but Quantum was the name I gave to the first 35" scale basses. The first Q6TB was very narrow, 2.5" at 24th fret. Different bolt-on models evolved: Quantum 5SP, SPX, then SPi, which is what the current bolt-on Quantum was originally called. I'm sure it's been posted around here before.
And to the person from Chicago looking to try a 5-string, the Chicago Bass Club has one of my walnut over ash 5-strings in stock. Same basic beast as the Q5SPi, but .25" wider at the 24th fret.