Ok so the exact stuff in this bass is EMG JX pickups, the single coils are VERY good at cutting through a guitar-heavy mix (which is why Mark chose them) and since they are the X series they are more aggressive than the standard EMG J pickups (not sure the exact model, but it is the JX pickup in a 6-string soapbar housing so I believe it would be EMG 45JX pickups) Mark also uses these pickups in his new signature W*****k bass for the same reason, the punch and cut through.
These pickups are coupled to an EMG BQC preamp which is very flexible (bass, treble, stacked mid and mid frequency sweep), as well as enhancing how articulate the pickups already are, EMG works well with EMG. Mark also boosts his high mids significantly (I'd say about halfway) because that's the sweet spot for articulate cleanish growl with these pickups, keeping the treble flat and sometimes adding a little bass or low mids. He plays through a GK amp which also adds to the growl, not sure how much of his EQing is on the bass and how much on the amp, but I'd say its a mix of both. He seems to blend just a hair towards the bridge pickup.
The long and short of it for stilla: The SSD pickups in general are weakish and middy, but kind of muffly from what I've heard. The first step would be to upgrade those to active EMG pickups. Some EMG pickups and preamps have "quick connects" which don't require soldering, but some do require soldering, I prefer passive pickups mostly myself so I don't have a lot of EMG experience, this Legend is my only one. My advice would be to try pickups first to get the most of the bass, you can always run the onboard preamp flat, and EQ the bass through your amp, then upgrade the preamp on the bass later.