itu

I might call this a three band semi parametric eq. Good looking graphs, by the way.

How about experience?

1) No. 2) Yes... partially. 1) 4 ohms here equals 2x8 ohms in parallel. If you wire them in series, you get a 16 ohm speaker. You could try this. You decide whether the divided power is enough for you. A 300 W amp pushes 100 W (1/3 of the total power) to the 16 ohm cab, and 200 W to the 8 ohm cab. Depending on the sensitivities of the speakers, the loudness of each cab may be whatever. 2) If you use one element only, you have an 8 ohm speaker. It would be feasible to replace the other element with a wooden plate. Otherwise the element with no connection would be a spring like component in the system. That would raise the lowest reproducible frequency (think it like the cab would have a leak). Note: The reason for the parallel connection (in cabs that include more than one element) is that if one element burns, there's still the other(s) left. A very good feature if you have a valve/tube amp because it always needs a load of some kind.

What PDF?

At all? I would open it - carefully - to find the issue. WD40 to the axle could help to turn it. Or not. If the latter, you have to open the case. That centre detent mechanism is the first to be opened (after removing the pot from the bass). You can see it has been hit with a chisel (two grooves at the end of the axle). Try not to remove any material, and you may be able to put it back. Opening the pot casing is usually the easiest part, if you do not ruin the metal parts. Take it slow, and put all parts to the same order as you remove them. More table and more light will be helpful. If all parts are in good condition, rebuild the pot. Use deoxit to clean carbon tracks. Excessive oil is bad, and does not belong to the tracks. Only to the axle. Make sure the contacts are functional. You should have found the issue that prevents turning by now. If something is broken, find another pot from the same manufacturer, and take spare parts from that. Good luck with your journey.

Well done! Congratulations! ...and thank you for sharing your skills to us mortals.

@Leon C 2 did a triple exposure. I have managed double so far. After reading this thread through, I am making my own conclusion (this is about nearly any bass there is). There are quite a lot of people trying to find a bass to their string set. So many complain about "weak B" or any other not so well sounding bass string or the bass itself. I suggest that if a nice looking bass was found, it should be treated with several string sets. Yes, it costs quite some (but the sets can be saved for future use). But if the sound will become alive with some set, the investment has paid itself. Most of my basses are strung with light D'Addario or GHS strings. They work really well for me. That B is a common nightmare, but other strings may present issues, too. Multiscale may be ergonomic, and may sound different. But if the strings do not support the whole bass, those extra inches do not mean a thing.

That pot clearly has a centre detent mechanism. May be downright impossible to find similar. Is there something wrong with it?

What's the difference between Chuck Norris and @Bilbo? @Bilbo can transcribe twice as fast as Chuck Norris, the man who has counted to infinity twice.

Sorry if I mislead you: Preamp outs are lo-Z. Therefore the summing amp could be done with a simple resistor based single. Inputs are pretty high, but for that piezo, it has to be very high, as you stated. Otherwise the input impedance would cut lows.

Jfet sounds like a hi-Z preamp, maybe a piezo pre. TL061 is only a single, could it be a summing amp? (All preamps are lo-Z, and could be summed via resistors.) Martin sure could open up the specs.

Or lower side for dry, and higher for wet. I split the band at around 400 Hz. Worth trying, although takes some time to find your own sound.

No no, this is a royal scam. A true bassist can not play and sing at the same time. Please leave this chat and go do your thing with King and Lee. (In other words, well done!)

Beautiful pair in lovely colours. Would you compare them?