The Quantum Bass is arguably the most famous bass made by Modulus Guitars next to the esteemed Flea bass. However, Modulus is also known for making a few variations on their traditional designs, such as the Flea jazz bass, the TBX basses, basses with upgraded hardware and electronics options and, perhaps the most unique and intriguing production run of them all, the Quantum Turbo.
The Quantum basses exist mostly as 5 and 6-stringers since far fewer 4-string models were produced. One could speculate a number of reasons for the lack of 4-string versions, one being that most people perceived the main strength of the Modulus graphite neck design being in the clarity of the low B-string and other detuned string tunings. This is a trait that I can whole-heartedly attest to. The "sweetspot/turbo" configuration is likely almost as rare as the 4-string Quantum and they were built in 4, 5, and 6-string models.
The turbo configuration places two dual-coil soapbars so that the neck pickup is lower on the body than the standard Quantum, near where the Music Man stingray sweet spot is and sandwiched right below it is the bridge pickup with only about 1/2" of space in between them. Together in equal blend they give the bass a big command of the low-mid range frequencies, being very throaty yet smooth and articulate with a slight bump in the upper mids, right around what sounds like 700-800hz. The neck pickup soloed produces a punchy voice very similar to that of a Music Man Stingray while the bridge pickup soloed gives a very burpy, thinner, jazz bridge pickup tone. For reference I use DR Lo-riders ss 45-105 strings.
The one I own is from 1996 and is especially rare because it is a 4-string Quantum Turbo (pictured). It came stock with Bartolini pickups and NTBT 2-band preamp, neither of which have I ever been a fan of. The pickups had too soft and polite of a tone to them. Almost "pillowy" and without much definition. Perfect for smooth jazz, dub or the like. Maybe some blues. But not for my style. The preamp, likewise, wasn't very flexible and the overall sound I got out of the bass discouraged me initially from playing it in my band as I like a strong, defined, aggressive tone for our style. However, the incredible feel and playability of the bass combined with the piano-like sustain as well as its sexy looks made me care enough to try swapping out the pickups and preamp.
I ended up going top shelf with a set of Nordstrand big splits and a John east u-retro deluxe 4-knob preamp. With these transplants it suddenly came alive. The highly overwound split single-coil pickups (in emg35 covers) completely opened up the Quantum Turbo's potential to "bark" and "snarl" if I dug in hard, or sing richly and clean as a whistle if I played softly. But even though the dynamic capability and tonal intensity of the bass were enhanced, it still had a fairly limited usable tonal range, isolated to the lower mids. Adding the U-retro preamp broke that barrier down completely. Now the tone can be sculpted into an incredible variety of fine tuned shapes. The bass frequencies can be boosted a ton if I need it, I can add or subtract treble all I want and I can completely sculpt the mids to my liking which is paramount since the mids are the Turbo's natural territory.
Overall I'd say the Quantum 4 string is an incredibly solid, very easy and fast bass to play with sustain for days which is even further enhanced by the upgraded 2tek bridge that mine was built with. The Turbo configuration has a particular tone range that will either make you love it or leave it. With the stock bartolinis that came in them at the time, they are not very tonally flexible but they may be just what you're looking for if that's your style. However, in spite of that, I can attest to their potential for versatility with the right electronics. They are a unique beast and if you can get your hands on one in good shape and you find the sound pleases you, don't let it go. It's a unicorn.