Note that a large part of the MusicMan sound comes from the preamp. MM preamps are not transparent, but rather, they impart their own voicing on the pickup. This makes up for the inherent problems of running two low impedance coils in parallel. It can be noted however, that many modern pickups in the MM pickup casing are not wound in the traditional fashion. From what I hear, there are many higher impedance pickups voiced for series operation.
Preamps can offer five advantages over passive systems.
First off, input impedance is constant. This means that changes throughout the circuit, such as plugging into different amps, will not affect the resonant circuit of the pickup coil.
Second, output impedance is lowered. This can be advantageous, because as the resistive component of the output impedance is lowered, the frequency cutoff of the filter formed by the parasitic capacitance of your guitar cable increases, giving you less "tone suck" from the cable.
Third, many (but not all) preamps provide some form on onboard EQ. This can be useful for tone shaping. You can do the same thing from other gear elsewhere in the rig, but many find it convenient to have the controls right on their bass.
Fourth, some (but not all) preamps provide a gain boost. While it is always important to consider your gain structure throughout the rig, and find the balance between the SNR/headroom and high output, it can be desirable to boost the gain of some pickups.
Fifth, though it's a rarity with most commercially available preamps, some provide multiple buffered inputs. This is useful for a variety of purposes, such as independent control over pickups, and output level-matching.
Preamps also have their disadvantages.
First off, though I find it absurd, myself, a great number of bass players tend to freak out over batteries. A properly wired preamp draws between hundreds of microAmps and a few milliAmps, which yields quite a bit of battery life, but nonetheless, there is always the thought that something could go wrong and leave you with a dead battery.
Second, since the input impedance remains constant and the output impedance is lowered, the tone is often a bit brighter than in a passive setup. Many people attribute this to some sort of "active sound," but in many cases it's simply the nature of the the wiring, and can be compensated, to a degree, to produce a more "passive" sound. If the pickups feed into the preamp with a minimum of resistive loading, you can add resistors to simulate pots in a passive system. You can run a small value capacitor across the input stage to simulate cable capacitance, as well. If the bass is still too hi-fi and sterile for your taste, you might consider a more darkly voiced preamp.
Third, there are theoretical concerns over the headroom and SNR by adding gain stages to your circuit. This really isn't a big deal to most people, as you usually can't even hear the difference.
Note that a bass can be wired for both active and passive operation, by keeping controls of an appropriate value for the pickups' output impedance before the input of the preamp, and then switching the preamp in and out of the circuit. This is my personal preference, and it's what I would do in this case. You get the best of both worlds.
