I am in the process of building nearly the exact bass the OP describes. Running it through combinations of filters and processing located in racks and panels, much of which is DIY. The bass is being routed with a universal pickup cavity to accomodate any number of pickup combinations. Control cavity is rear routed and front routed. The front hole to be covered with simple 2" wide aluminum plates that can be replaced with another 2" plate in the event the one on the bass get's drilled witth too many holes from making changes. The plan being to run the pickups straight out and hitting processing gear outboard. My foot will do all of the pickup mixing with a sweep pedal connected to panning/fading modules. The EQing is done through modular synth filters that have variable states ... (LPF, HPF, BPF, Notch).
The body has been ordered through Warmoth. A solid maple "G4" shape with a "bathtub" pickup route and double batt boxes in the event I end up using active pickups.
I've posted these pics a lot, but for those that haven't been exposed to them yet, here's SOME of the filter array. I have seven more modules enroute as I write this. Using Baer ML112 and a Mackie 1400i for monitoring. The Baer is here now, the Mackie is paid for ($220 shipped) but not here yet.
Here's the super-fresh Baer ML112 bass cab.
I'll be doing different signal routings. Sometimes going in stereo, other times in stereo-mixed-to-mono, other times both pickups mixed to mono and processed in mono. In any case, I can use my foot to mix the pickups with a sweeping "gas pedal" modulator. I also have gas pedals to change filter freq, filter resonance, pan position, all per-pickup.
I also have four tube mic-pres, and several DIY overdrives that may be inserted at any points in the signal chain. Going with modular synth gear opens up the entire signal chain to you in ways your brain hasn't thought of yet. Two filters can be parallel while the last one can be in series. All four filter can be in series. All four filters can be in parallel with overdrives applied to each one in differing amounts. So you can distort (let's say) just the upper mids while the low end remains totally clean. Whatever. If you can ~think it~ you can probably ~do it~. Noise? Nope ... this is pro-level synth gear. If output impedance becomes an issue just stick a DI at the end of the chain. The entire system is surgically sliced wide open.
The synth cabs are purpose built by a fella in Canada. $122 shipped to Arizona. However these synth modules can also be rack mounted if an adaptor is used. Here's what I used for the modules before I got the synth cabs made. It's a 12u rack that I made simple rack adaptors for out of 1/2" x 1.5" oak trim stock I had left over from another project.
I'm going full rack mount using a Gig Rig that I have been tripping over for ten years. The modules will face ~up~ at me being mounted where a mixer board would normally go. The Mackie will sit in the lower rack portion along with Furman power conditioner and a few other niceties. The Gig Rig is castered so it rolls very nicely, it'll make this setup uber-convenient. The covers fit over the synth modules without having to remove any patchcords.
I'll have more modules and the two rack adaptors this week. More pics soon. I plan on doing a full-on info thread that will include audio demos very very soon.
This rig was less costly than buying a 4x12 and an SVT by the way.