I did some quick reading, is shelving more or less the same as using a low pass/high pass filters?
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No, they are not.
Shelving Eq's give you a specified boost and cut for a desired frequency range.
Filters just cut at a specified slope and frequency.
There are differences in the frequency responses as well as the phase response.
A quick google search for frequency response of shelf EQ, high pass filter, and low pass filter will help.
There is such a thing as filter based EQ's which uses bandpass filters each with its own gain control for boost and cut.
Electronics | AC Guitars This is another guy who makes a filter based preamp like you describe I have one in my 6 string and while the learning curve is steep it gets some neat sounds. Good luck with the build if you go that route I'll be watching. I wanted to do this to my all stock 65 Starfire but my Luthier friend thankfully talked me out of it. I see your bass is kind of a mess though so nothing lost there. I'm considering getting a new 2 pickup starfire to modify with though. Good luck 
Electronics | AC Guitars This is another guy who makes a filter based preamp like you describe I have one in my 6 string and while the learning curve is steep it gets some neat sounds. Good luck with the build if you go that route I'll be watching. I wanted to do this to my all stock 65 Starfire but my Luthier friend thankfully talked me out of it. I see your bass is kind of a mess though so nothing lost there. I'm considering getting a new 2 pickup starfire to modify with though. Good luck
I looked into that one for a bit but it's complexity and price kinda turned me off. I still dont fully grasp how it works.
IF I had extra money to throw around I'd love to try out that system but for now I'm pretty set On (for now) a 3 band semi Parametric is more than what I really need.
LoveThatBass
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I made an outboard power supply and use a Mogami 2 wire with shield and stereo plugs to connect the bass to the external power unit velcroed to the top of my amp head. That same cable can be used for standard basses also (not a problem and works well). I put my external power unit in a small cast aluminum box and an 18 volt wall wort plugged into it. You can also use a 9 volt wall wort if needed.
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Velcro, wall warts? Not my cuppa, but definitely another workable approach for sure.
the circuit I linked to from rod Irving's site needs only fairly minor mods to work as a 3band sweep eq.
unless you have active pickups, you'll need to increase the input z by altering r1. I'd remove the existing treble control and tweak the two mid controls to work for your needs.
be aware that this type of sweep filter will change its band width as you sweep. the lower the freq, the higher the q. in practice, this is fine if you keep the range to no more than a couple of octaves, as rod himself has done.
this is an ideal project to learn spice. Ltspice is a free download and is more or less the hobbyist's choice these days.
you will also need to bias this to work with a single 9v supply. you can either split the battery to float above and below earth (+/- 4.5v), but you'll need a Jack with a separate switch. that's fine for a front mount or plate mount, but not an option for a barrel Jack.
you can also go with a conventional biasing setup. a couple of resistors as a voltage divider to create a virtual earth at 4.5v. this way you can use the standard battery-neg-to-earth with a stereo jack arrangement. but you'll need an output coupling cap.
unless you have active pickups, you'll need to increase the input z by altering r1. I'd remove the existing treble control and tweak the two mid controls to work for your needs.
be aware that this type of sweep filter will change its band width as you sweep. the lower the freq, the higher the q. in practice, this is fine if you keep the range to no more than a couple of octaves, as rod himself has done.
this is an ideal project to learn spice. Ltspice is a free download and is more or less the hobbyist's choice these days.
you will also need to bias this to work with a single 9v supply. you can either split the battery to float above and below earth (+/- 4.5v), but you'll need a Jack with a separate switch. that's fine for a front mount or plate mount, but not an option for a barrel Jack.
you can also go with a conventional biasing setup. a couple of resistors as a voltage divider to create a virtual earth at 4.5v. this way you can use the standard battery-neg-to-earth with a stereo jack arrangement. but you'll need an output coupling cap.
oh, and you may want to use reverse audio taper pots for the freq sweep controls. I'd source them first before you model the circuit. linear pots will work at a pinch, however...
I looked into filter based eq's briefly at one point. I hope I can explain it well enough...
Simply put, for each frequency band (low frequency, high frequency, low-mid, mid, high-mid,...) there would be a band-pass filter. Each filter would then need some sort of gain/level control for boost cut, though
there might be a way to incorporate boost/cut control into the filter. As I recall, each filter would need to be in parallel and the output of each would then need to be summed together to reconstruct the entire frequency range.
I saw someone liked this post of mine from 4 years ago. So I wanted to make an update and say I've been working with the Empress zoia and a custom midi pickguard I made for my bass. Using both I'm able to use what ever kind of filters and modify it on the fly.