What's making this noise? Please help me figure it out!

[anotherjim]

I'm not sure what you already tried, since I or anyone else here really knows if you've taken many of the suggestions here seriously/literally and how or where you tried them. I know you say you tried RC filter but have you made and fitted the one I just recommended? Did you screen the wires & cavities? I'm trying to tell you it should be done and kept whether or not it fixes this particular noise problem. And what about the guitars pickup cavities - are they screened.
Proper filtering requires taking measures you may never know you need.

The datasheet I posted of the converter chip shows a 400Khz oscillator.
The inductors are cheap - try them!
There's no reason not to try inductors in series with the negative power wires as well as the positive.

[Thebrainless]

I'm not sure what you already tried, since I or anyone else here really knows if you've taken many of the suggestions here seriously/literally and how or where you tried them. I know you say you tried RC filter but have you made and fitted the one I just recommended? Did you screen the wires & cavities? I'm trying to tell you it should be done and kept whether or not it fixes this particular noise problem. And what about the guitars pickup cavities - are they screened.
Proper filtering requires taking measures you may never know you need.

The datasheet I posted of the converter chip shows a 400Khz oscillator.
The inductors are cheap - try them!
There's no reason not to try inductors in series with the negative power wires as well as the positive.

I understand, I haven't shielded everything yet, just tried on the circuits and saw little to none improvement, so I thought it might be avoided since to shield every cavity is a PITA, but still I've ordered adehesive brass tape to eventually do it...
Your filter I haven't yet tried, I've tried simple Rc filters like this



With always 10 uf 50V capacitor and various resistors to change frequencies (I've posted video results earlier anyway).

I just think "if the only variable I've added to the equation is those circuits, the problem must be found there, not in the bass shielding or preamp or whatever"
I've tried shielding the step up circuit by enveloping it in an aluminum case isolated from the circuit and going to ground, it changed nothing, so again why bother keeping it?
Maybe my approach here is wrong, I don't know, but I'm going with what I try and learn, and if something useless...
Anyway, as I wrote before, Iìm waiting for the shielding tape so I'll be shielding the F out of the whole bass, I know that won't hurt so it's something I'll be doing for sure.
I've looked further into voltage regulators and stuff, so I'm waiting to get a 7818 chip, to try if putting it to regulate the voltage after the step up converter might work.
It might be an overkill, or at least a waste of current, cause 7818 has a drop of around 2V, so the step up must be set to around 20V in order to get a clean and stable 18V after it, that means lower current available from the battery, if I got it right, but it's a little price to pay in case this will work...
Then I'm waiting for a circuit which includes both lithium charger/protection and step up converter in the same small pcb, maybe it'll work better with less noise? I'll see...
If I think about all the money spent on circuits and stuff until now, it'd have been the same as if I bought the fishman battery pack, but where's the fun then?! 
Oh, talking about that, here's a close up of it's circuits, but it's just one side and it's all smd, so it's pretty impossible for me to understand what's going on



I'm having fun anyway, kinda frustrating fun sometimes, but still I'm learning a lot...
So let's wait for all the stuff the arrive then I'll try those other solutions!

[Thebrainless]

VICTORY IS MINE!!!

I've managed to solve the problem, now the bass is more silent than before, with no square signals, no strange noises, even the normal noise to signal ratio has improved.
All it took was to use the bigger dc dc converter that has less noise on its own, and put a voltage regulator right after it, before hitting the preamp.
So It's lithium battery - charge/control circuit - step up dc dc converter - voltage regulator circuit - preamp.
Here's the voltage regulator circuit:



It's a simple circuit found online, nothing crazy, but I thought that a stable DC could be the solution, and apparently it is!
The only downside is the 7818 IC works with a higher voltagethan 18 V to give a stable 18 V on the output, so the converter is set to 19.3 V in order to have a stable 18.1 V, there's a loss of 1.2 V , so apparently I've got 590 mah from the battery instead of 630 mah, I've lost 2 and a half hours of playing, but gained a clean and stable signal, so it's a great bargain IMHO!!!
I've managed to cram the circuits in the bass battery cavity, and the battery sits just above the potentiometers in their cavity, so from outside it's a clean job, If you want photos lemme know, I could also make a video about this journey as well, since it was my goal from the beginning....
Thank you everyone for all the infos and suggestions, even though you guys are a little too worried about shielding  but I know it helps as well.
Now the next project will be....

[GibsonGM]

I'm surprised it requires that much regulation, but then - I'm not surprised   Great work!